The idea of not paying for a voucher is certainly a good one when it comes to these agencies (won't name names!). If you must buy a voucher make sure you get it direct from the FTO themselves but even better, do what modelman suggests. The problem doesn't so much lie in the FTO going bust but in your mate realising after 6 months that he hasn't yet been up and then the good ole UK weather interfering - next thing you know the voucher has expired and everyone gets all fizzy over the phone.
Seen a few TL's go belly-up for that reason alone and at the end of the day it's the person buying the pressie (ie; You!) who loses their hard earned.
VFE.