how exactly would i go abo ut this? i'm us9ng Duke Patricks Nosferatu class vampiresand i'm not sure where exactly I should go in the CS and if i sho uld launch the CS with or without OBSE? Do I need to tweek a script? is this legal?

A) What are you trying to do
B ) Get Permission if you want to release it.

As long as it's on your own computer and doesn't affect others, there's little others can do about it, so legality shouldn't be an issue.

That said, you can do whatever you want with a mod (of course, redistribution is frowned upon, but you're doing something just for you, on your system).

I have no idea what the mod involves, but I imagine you start up the CS, load up the .esps you need, and look at the scripts, whereever they are. Sorta limited in experience with that.

How do you use OBSE with the CS? I never heard of that... Don't you just write the script, and if you load the mod without OBSE in-game, it just won't work/crash? Since it will try to use unknown functions.

As long as you keep the mod for yourself, it is fine, but it never hurts to inform the creator.

To edit it, you just make the plugin the active esp when opening it in the CS. And if the mod in question requires OBSE, you must launch the CS with OBSE as well. If not, it will not compile. As for whether you must tweak a script, that of couse depends on what change you want to do.

http://www.tesnexus.com/downloads/file.php?id=15236

the mod in question is here. I want to tweek it to remove some of the new weaknesses added to NPC Vampires in order to put it more in line with another mod i'm using

Well do you want to do? Explain this in detail, so we know what to tell you. If you want to change the scripts of the mod, you have to use a completely different section of the CS than when you want to add or change some of the spells or abilities the vampires get.

Also don't forget that many people might not know the details involved in that mod. For example, you're asking us if you should launch the CS with OBSE; so please tell us first if the mod even needs OBSE! Or maybe the changes you want to make will require OBSE? We don't know necessarily know these things, so tell us.
We also don't know if you need to tweak a script, because we still don't know what you want to do.

In general, you need to load the CS with OBSE if you want to add OBSE scripts to a mod, or if you simply want to modify a mod that is already dependent on OBSE.
Even more general: It really doesn't hurt to have your CS always load with OBSE. Mods will not become dependent on OBSE only because you started the CS with it.
To open the CS with OBSE, make a shortcut to the obse_loader.exe. Then right-click on that shortcut and look for the line where it says the path of the obse_loader.exe. Here, you simply put a "-editor" at the very end of it. In my case, it looks like this:
C:\Games\Oblivion\obse_loader.exe -editor

If you double-click on that, the CS will open instead of the game (which is always fascinating to me when I think about it - one exe opens two programs!).


Edit: Okay, that's a good start. So the mod requires OBSE 18, and you want to tweak some of the "weaknesses of vampire NPCs". While you won't do anything that requires OBSE, the mod itself requires it, so you should load the CS with it.
Now you need to be a little more precise what kind of weaknesses you mean. Do they have things like "Weakness against Fire 100%" or other abilities like that?
If so: These kinds of weaknesses are Spells. To be more precise, they are Abilities.
In the Object Window, you'll find the entry "Magic". Click on the "+" next to it, then on the "+" next to the entry "Spells". Then, click on "Abilities". Those abilities that have been edited (or made) by Duke Patrick will have an asterix (*) next to it. Their EditorIDs will probably start with some sort of a tag, so they'll all be in one place.
Now all you need to do is find out which ones are added to NPCs, and then edit these by double-clicking on them. (Look at cs.elderscrolls.com for more information on how to make/edit spells or abilities. Simply search for "Magic".)

I want to tweek and remove some of the added drawbacks given to NPC vampires. namely the sun damage but i'm not sure how its added to the vampires. it is done in a very roundabout way for compatibility reasons meaning the new traits are.. I think added by scr4ipt but i'm not sure