Having never played through this DLC yet, do I miss out on any special transformation scene or anything if my character is a vampire before starting the main quest? Or is it just nicer to become a vampire within the natural flow of the main quest? Reading only the light spoilers I get a sense that you sort of start doing things for the Dawnguard faction first. Will being a vampire mess with that? I don't want heavy spoilers, but light are fine.

Basically, is there the "authentic experience" and the "well, you can do it that way but it's a little more cheesy" when it comes to already being a vampire?

The simplest way for me to describe it would be like this:
Pre DLC you're "Vampire 1.0"
In the DLC you can become "Vampire 2.0"
... but to begin the DLC you'll need to cure your vampirism. They'll just turn you away if you don't.
(I think...I know that later on in the quest-line they will turn you away if you want to continue down the Dawnguard side.)
Later there will come a fork in the quests. If you choose to continue on the Vamp side you'll have the option to get it back from them, and become "Vampire 2.0"...which is really really fun

I know you can be infected with sanguinas vampiris (or whatever it's called). I was when doing the first dawnguard faction quest (where you find serena for the first time). I turned into a vampire when fast travelling to the Northwatch keep, so I was a vampire just minutes before meeting Harkon for the first time. Harkon can turn you into a vampire Lord, though. He will make some comments about you being a "lesser vampire" if you're a normal vampire.

If you choose to become a vampire Lord, you are thereafter part of the vampire faction. If you instead decline Harkon's "gift" you can return to Isran (the DG leader) and instead choose to side with the Dawnguard vampire hunters.

This certainly wasn't true for me. I've been a Vampire for most of my game before Dawnguard, and continued to be a Vampire. There are some whacky inconsistencies if you want to be a Vampire in the Dawnguard, but you can go the Volkihar route, starting in the Dawnguard and then heading to the castle without ever needing a cure of your initial vampirism.

I would try both routes: non-vamp and vamp. I just tried the vamp route when meeting Serana. Very different dialogue than when you are not a vampire.

Serana asks if her father sent you to get her. Then when asked "How did you know I was a vampire?", Serana replied, "What, you didn't think I don't know my own kind? I could smell you way before my eyes were opened."

Than when speaking to Harkon, the dialogue changes too. Here is a video of a http://www.youtube.com/watch?v=PzesNUyW0g4 before given the power of Vampire Lord.

No they don't. Nocturne was a Stage 4 Vampire Assassin when he started the Dawnguard DLC. He walked right into Fort Dawnguard with no problems. Even the Leader of the Dawnguard was clueless to his nature. They gave him the quest to explore the cave for the Vampire artifact, and he took Serana back to her home in Volkihar.

Never once did it become a problem that Nocturne was a lesser Vampire before starting the DLC.

Now Nocturne is a Vampire Lord and Assassin. While he has not taken care of the Harkon issue, he plans to rid himselfof this most annoying thorn very soon indeed.

I was so frustrated by the complete lack of logic to being Vampire who wants to stay a Vampire and join the Dawnguard.

Good point.

You can't accept the gift if already a vampire supposedly. I've read many people say that.

The vampire faction will still accept you, but no VL form.

The dawnguard rejects many people as a vampire until cured. The ones who stayed in in their game I don't think we're supposed to but it worked out for them.

A lot of people upset for being rejected as a vampire ironically

That's not true, you can become a Vampire Lord if you're already a vampire. Harkon will comment that you have a disease but are no true vampire and will correct the situation. As for the Dawnguard, you can initially join them as others have said, and receive the quest to free Serana, but once past that point you will either have to cure yourself or accept Harkon's offer.

^This was true for me. I went from being a Vampire to being a Vampire v2.0.

In a nut shell it doesn't matter what you are beforehand if you wish to progress down either questline be it vampire or dawnguard you are forced to be exactly what bethesda wants you to be for that side, vampire = v2.0, dawnguard = mortal.

IMO it's a load of [censor] and on the PC at least I can wait until a few more mods come out and crack this [censor] idea of bethesda's to bits.

I always found that to be hilarious. I'm working with vampire hunters and yet I'm forced to escort a vampire with an elder scroll mind you, back to her coven of vampires. Yeah. Even Isran was like "And you just escorted her back to them?". Ugh. I couldn't kill her or anything.

But hey logic, who needs it?

Seridur and the Order of the Virtuous Blood would like to have a word with you.

Are you kidding me? Those bunch of buffoons who couldn't tell their own leader was a vampire, they are about as threatening as butterfly in Skyrim.

Ah! Well, to be honest I was making an assumption based on the fact that later in the quest chain they turn you away if you're a Vamp. I just assumed that was standard from the beginning.
How strange that it's not....

I always thought that too. Terrible vampire hunters they are.

Hence why I believe the vampire main quest line flows better with serana(I mean dovahkin is a vampire for the title picture for dawnguard) I personally think dawnguard should of got their own npc for the DLC.

Isran is a turd. First he berates you for not taking out an entire coven of vampires all by your lonesome, and in the next breath, he asserts that the Dawnguard need to build up their ranks before they go after said coven of vampires.

Although I particularly enjoyed it when he told me "Avoid sleep. Sleep is for the weak." just before he tucked himself into bed.