I'm curious to know what you found limiting about Vital's free (as in beer) offering. Other than fewer wave tables, presets and a limited number of text to wave table usages per (Day? Week? I'm not sure what the timeframe is) I am certain there is no difference compared to any of the paid offerings.

If you're interested in soft synths that are FOSS, you might want to check out these as well:

Vitalium (a completely FOSS fork of Vital, in a similar vein as Chromium compared to Google Chrome

Helm (developed by Matt Tytel as well, just like Vital) Surge (XT)

Zyn-Fusion (a FOSS additive synth)