Dennis,
I would strongly suggest you simply do the minimal work to port your Vignette Tcl to vanilla AOLserver first. Then, determine what value you'd get out of OpenACS, vs. just continuing to run and operate your Vignette apps under AOLserver alone.
I have plenty of Vignette experience (heavy R4.0 through V/5 5.6) as well as a fair amount of AOLserver experience. 😊
If you want to discuss things in more detail privately, feel free to email me -- mailto:dossy@panoptic.com.
However you decide ... good luck!
-- Dossy