I've tried removing one of the files, actually both but one at the time, but VDub still refuses to start...

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AudioSlave

Then you have an other problem

By the way, there is no link between vdub and avisynth so I don't see how a plugin can make it to not start. Explain step by step what you are trying to do and where it fails.

When I'm trying to start MovieStacker 2.1.0 it says:
External exception E06D7363- Loading avsWarp.dll failed!

I don't know what that file is and I don't recall ever using it?!

* * *

When I'm trying to start VirtualDubMod 1.5.10.1 it says:
---------------------------
Microsoft Visual C++ Runtime Library
---------------------------
Runtime Error!

Program: ...VirtualDubMod 1.5.10.1 (All Inclusive)\VirtualDubMod.exe

This application has requested the Runtime to terminate it in an unusual way. Please contact the application's support team for more information.

* * *

Both these errors started occuring after I put DGDecode.dll in the AviSynth plugin folder...

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AudioSlave

Trust me, this in not linked.
Do you install or uninstall something else recently ?
YOu can try to reinstall avisynth but I have doubt this will fix anything.

Note: I don't have any avswarp.dll on my disc either.

Nope, that's the really wierd thing!

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AudioSlave

I added some lines...

Empty your plugins folder and try again. Then add only those plugins that you need.

MovieStacker 2.1.0 access frames produced by AviSynth through avsWarp.dll.
This dll has some bugs and may crash depending on the avs script problem.
So I guess the problem is really with your script and/or plugins used...
Aren't you trying to decode a DVD2AVI 2.7x with DGDecode.dll (or the oposite) ?