You can try uninstalling the graphics card in your Device Manager and letting Windows install the driver. It likely uses an older driver than the geForce site -- and quite possibly a more stable driver for your purposes.
you going to struggle with an older driver I think as the 980 is the new architecture which is now 2 versions removed from what Vegas seems to like from back in the 1990's or whenever it was.
I'm running a 780ti + 980 which work great with all my other modern apps I use even the open source stuff works the 980 is slower than the 780ti with vegas (and a little bit in general) also I only now seem to be able to slect just the 980 in vegas, not sure where the 780ti has gone to everything else can see it and it's my primary, neither of them work properly with Vegas, I'm just lucky I've got a decent CPU setup which masks the fact some what and makes vegas usable.
If Vegas is your primary use you might be better off getting a cheap old second hand card to run it as a second card and keep the 980 for all the current apps.
Just a small follow up that may be useful, I was trying to fix why I could no longer select which graphics card for vegas to use and I did a full reset of the latest 444 build and both the cards reappeared however with 4k footage only the 780ti worked the 980gtx didn't the display was corrupt.
I remembered that previously I'd gone into the internal advanced settings and set the OpenCL Filter memory size filter to a larger value, I set mine to 3000 the 980 has 4 gig and the 780 has 3gig and the 980 started working. So far it's not like wow that's amazing but it is chugging along and vegas does utilise it a little ~5-10% so it's usable and stable so far. If you have a 980 and having trouble it might be worth trying that.
Interestingly neat video just picks them both up and gets on with optimising across both of them in conjunction with the CPU, Blender which is open source on opencl as well just goes great give me both I'll max them , you want to run another instance just on CPU at the same time, sure and hope your PSU is upto it :) so it is doable and the libs to do it are all out there.
I think I've said this before but I really wish other vendors would look at this method where you give the end user a real choice and optimise for the system it's running on not just vegas, just in general now multiple cards and cpus are becoming much more common.
Silly suggestion here, but it MIGHT work... rename the vegas exe to something different. It works with games. GPU makers can't make things perfect (IE can't fix mistakes in someone else's code). so instead that find the issue with a specific game (IE Doom 3) & have their code do something specific when THAT game runs using that file name.
So, changing the filename of Vegas to something else might work. IE change it to the Premiere exe's name.