SourceForge is strange. Still offers pack 37200 as "latest" although
v11 contains pack 37277. {x86-64: +1}
VirtualDub 1.10.5-prerelease (build 37277/release) does not appear with a visible window as long as plugins32\scripted.vdplugin is present.
Veedub64 1.10.5-prerelease (build 37278/release-AMD64) does not appear with a visible window as long as plugins64\scripted.vdplugin is present. (Unpacked to a different base directory, of course.)
I hope you did not hyper-optimize your plugin, my CPU is only a Phenom-II X4 (x265 refuses to use SSE3 on it, at most SSE2).