is the IVL studio worth the extra expense of buying another MFX card over using VST plugin's in the DAW?
Only if you absolutely require the effect and/or you require access to more than two reasonable onboard effects.
As Peter mentions....your money would be better spent on improving the technology you use within your DAW than dropping it on a second MFX card just for the IVL Vocal Studio.
OK, that said, there is actually a VST/AU plug-in that duplicates pretty much every function of the IVL, with one major exception: no live throughput. It requires input from a pre-existing audio file in order to function.
This would be VPlane's Vielklang. I got this plug-in through a beta program. After comparing it with the IVL, I can say that it sounds and edits about the same....but there is a certain je ne sais quois that the IVL has that the Vielklang can't match. I dunno...it just sounds smoother. And then there is the live input issue. On occasion, I love to plug my guitar into the D8B and run it through the IVL off to the DAW. Like recording with any effect, there is always a tendency to adjust your performance to "work the effect". This is something the Vielklang simply can't do.
If you
really need to have this particular effect, my advice is: go get a second MFX card. But if you want to get the general idea of what it is...download Vielklang's demo. All in all, the card may turn out to be cheaper than the Vielklang's $99 pricetag. Or not. YMMV.