I've been doing photogravure with photopolymer plates for the past year, and not found a vaccuum frame to be vital for my work. I use a big piece of thick plate glass and make sure my plates are cut cleanly and printed with a nice firm contact, and my results have been quite good. That said, my dot screen is reasonably coarse, so maybe my grain is hiding it.
Do keep a close eye on the edges of your plate though, they do flex and curl like the dickens as Keith mentioned. And please - dont drop them...

A (very) accurate UV exposure unit is far more vital in getting started.