So – alpha can be pre-multiplied or un-premultiplied.
Syphon makes NO adjustments to the incoming textures alpha channel, its up to applications to handle alpha. Sadly different applications can use different versions.
It sounds like one app is sending premultiplied alpha, and the other is assuming unpremultiplied.
Graphics is hard and annoying. I’d get in contact with the VPT people and see if they are assuming an alpha format. That looks like premultiplied to my eyes, and I’ve not seen that with any other apps, and recall I had similar issues in Max/MSP/Jitter without changing the blend mode in Jitter.