If you know how to setup a print job (bleeds, trim marks, ink coverage, embedding files, fonts, etc.) then a PDF should be adequate. I haven't had a physical proof for dozens of print jobs in more than 10 years. It's a pain in the ass couriering shit around, especially on tight deadlines. If you have doubts about some aspect of the job, simply print the PDF off on a high end colour laser printer.
As others have said though, Vistaprint is ass. If you want good quality printing, you're much better off finding a local print shop that does proper offset printing to work closely with.