When I first got into this business many years ago I had a laser Cannon printer using vellum, you couldn't tell me nothing back then, until I saw an article in a now defunc mag The Press I think that's what it was called. David Crain had this film called posjet using an epson 3000 and a epson 1450 doing seps and said you could almost get imagesetter type film for less, long story short I bought some film and the epson and been printing every since plus I still use one of cadlinks first sep program's for screen printers call Photoscript 7 still going strong. Pierre is right go inkjet, your still going to have a problem here and there but not the registration problems you'll have with vellum and a laser plus you want have to tile large images...largest vellum sheets I ever got was 11x17 inkjet 17x22 and much larger.
darryl