Boxing would help but it is better to fill it with a more solid material (like plaster) or just resin and figerglass the crap out of it to make it strong than to use foam. Foam, by its nature, is mostly air and will collapse in a vacuum.
I prefer the MDF and bondo method since the master bucks last quite a while and can be modified if you need to tweek a detail or two. If you do go that route it will not really save you any money over buying a pre-formed kit suit. The vacuum table (I made mine for about $100), the HIPS or
ABS plastic, the bondo, the MDF and then the various little things (tools and such) quickly add up to what an OK kit would cost. Plus it would take you months longer to make.
The only real reason to vacuum form is to make multiple suits. If that is your goal, then rock on! If you just want one suit for yourself, I would buy a kit or do a resin/fiberglass/pep suit as a cheaper alternative.