Please, no spoilers here.

Okey, about vampirism, The Companions and The Dark Brotherhood.
So this is what I want to ask you all about..

If I join The Companions, will I be able to join The Dark Brotherhood?

Joining The Companions as a Mage, is it rewarding in anyway? And will you be the only Mage in their guild?

When you have been turned into a vampire completely, will everyone act hostile against you / and if yes, will you gain some new friends that was hostile before when you wasn't a vampire?

If you become a vampire, will this have an negative effect for you - if you decide to start at The College of Winterhold?

These are some of my questions I can't find any answers to without reading into a page full of spoilers, so I ask here insted.

Also, I know by 99% you can do anything you want in this game.

But in a role playing sense - how good would a sneaking Mage fit into the Thieves Guild, in your opinion?! Would you be the only Mage-type character in their little club aswell?

And last question about vampirism, is it possible to control which stage you will stay on without cheats, mods or console?

Final question, is there any skills that is a -must have- for some of the guilds you join?

If you join the companions you will be able to join any other guild including the dark brotherhood.
As a mage it will be rewarding as in there is no restriction to how you kill your enemies, all though there is no rewards as in items that benefits your playstyle. The companions is a fighters guild. They use swords an stuff and they have lots of fur on them too, dunno why though

As a comlete vampire everyone will look at you as an enemy and will attack you at sight (guards) and civilians will run. AS a vampire you have quite many negative effects and some new positive ones. I have not been a vampire myself but i have read into it.

Hope this helped abit

I`m awsome and i know it!

companions have fur because (major spoiler) [hide]the inner Circle are werewolves, you can become one[/hide]
becoming a werewolf removes and prevents diseases, including vampiricy.
vampiricy has four stages. stages 1-3 you can go outside safely. at stage four you will be attacked on sight. to go back to stage one, drink blood. every 24 hours without blood makes you go up a stage. it is possible to become a "vampwolf" as i call it, on PC (and Xbox, but you'll nreed Skyrim on PC as well, or a friend with PC). use console commands to remove the resist disease effect, and get infected. for Xbox, you have to port the save (a ricky process, but not too hard), console command it and port it to xbox. i created a thread about this, look it up.
i think that some vampired will be friendly to player vamps, not sure

If I join The Companions, will I be able to join The Dark Brotherhood?

Yes.

Joining The Companions as a Mage, is it rewarding in anyway? And will you be the only Mage in their guild?

It's the same for any character, regardless of class. Yes, you'll be the only mage.

When you have been turned into a vampire completely, will everyone act hostile against you / and if yes, will you gain some new friends that was hostile before when you wasn't a vampire?

If you let yourself go all the way to the final stage, everyone will be hostile towards you. There may be a few who aren't like the Dark Brotherhood but nearly everyone will want your head on a pike. To my knowledge, you do not gain any new acquaintances when you become one.

If you become a vampire, will this have an negative effect for you - if you decide to start at The College of Winterhold?

No negative or positive effects, just so long as you don't progress to the final stages of vampirism.

What?

To be honest what is a vampire Mage doing in the companions guild? What's the RP angle? Are you a battle mage?

Define your character before you start and then follow through with how your player would react to situations.

Some ppl bend and change the RP as they go, I prefer to stick with a rigid set of rules, prevents me from simply creating the same type of character all the time.

i join them for the special power you get once you become part of the Circle, and for Aela. im a mage.

So only The Dark Brotherhood will give you any armor that a Mage could use, none of the other guilds will give you any armor or so except The College of Winterhold then?

Funny thing is, I did the quest to remove the werewolf spell, and even now when I walk in a town guards tell me I have fur growing from my ears.

What's up with that?

Illusion mage=Master Thief

probably a bug, a side effect... or you need to trim your ear-hairs.

I have reached stage 4 of vampirism, but I can't seem to figure out how to drink blood in order to get back to stage 1. How do you drink blood????

I have reached stage 4 of vampirism, but I can't seem to figure out how to drink blood in order to get back to stage 1. How do you drink blood????

Sneak up on someone that is sleeping.

sneak up on a sleeping person and use the activate button/key.

That's because the guards' werewolf comments are triggered by the completion of a certain quest rather than conditioned to only play when the player is a werewolf, the latter of which they should have done, but for some reason or another didn't. Maybe it wasn't possible for them to have something that detects whether or not the player has lycanthropy? That would be kind of a strange excuse. Maybe they could fix this in a patch or DLC or something. There are enough complaints about that for them to possibly consider taking action.

I still can't seem to sneak up on anyone sleeping. Is there a certain trick in order to drink blood?

Order lydia (or any other follower) to sleep in a bed, stand next to her and simply drink her blood.

Just a heads up, in case you didn't catch the info in one of the posts above and are thinking of combining the vampire/Companions thing... you might be able to join them as a vampire, but you will not be able to progress very far in their questline and remain one. Joining them leads inevitably to becoming a werewolf unless you do the first couple of quests and then walk away without finishing out their storyline. Becoming a werewolf will cure your vampirism. You can, however, cure yourself of being a werewolf after completing the Companions stuff, and after that it should be possible to become a vampire again.

Is there any specific spot to catch someone sleeping or any special time?