will all vst plugins work with Splat in win 10?

No problems here.

I can't think of any reason why they wouldn't. Except during its initial installation, a plugin primarily interacts with the host, not the O/S.
 
In the past, most plugin compatibility issues involved Windows permissions (such as when they needed to write to a registry hive or folder that became restricted or renamed). AFAIK, Win 10 does not impose any new restrictions beyond those previously implemented in Win 7 and 8.
 
As long as the host is compatible with Win 10, plugins running within it should be OK. Any plugin that installed OK under Win 7 or 8 should also install OK under Win 10. (Possible exception is if you have a very old plugin that required installation under XP- or Vista-compatibility mode, which have been removed in Win10.)
 
Most of the issues we'll be seeing will involve drivers. Unlike VSTs, drivers do interact directly with the O/S at a low level so there are more opportunities for failure. Even with drivers, though, most of the incompatibilities we've seen so far have not been with the drivers themselves, but rather their installers. Fact is, nearly all Win7- or Win8-compatible drivers should be compatible with Win 10.
 
I think we're a little over-anxious about this as a result of painful past experiences with other Windows migrations. However, this one should not be nearly as traumatic as the the move from XP/Vista to Win 7, which was a significant paradigm shift by comparison.
 
Of course, I could be completely wrong about all this. Personally, I'll be installing Win10 on a non-essential machine first.