Is it even possible?
 

I'm running 3.8.x with vBadvanced along with several mods such as the video directory, opt-out forums, vbseo site map generator, flash chat, check4spam and a couple others.

Is it even conceivable to think about moving to 4.0 in the next decade and still keep my functionally.. especially vBadvanced?

Maybe a good discussion topic, how do you get from here to there with an established forum without loosing all the features you and your members relay on day in and day out?

You should ask over in the vbadvanced forums about whether they will be doing an upgraded package.

As for your other mods, some will be upgraded, some won't. So, your options if they aren't upgraded are to either upgrade them yourself, or pay someone to upgrade, or do without the mod, or replace it with something else.