Say, I've got an idea. Maybe stage 1 would just be the roll and the stat bonus, but stage 2 would be stage 1 HP + New stat bonus, with stage 3 being stage 2 HP + new stat bonus... as in...

Say I started off with 4 VIT (and assume that's the stat we're working with), so, no bonus...
I roll a 3, so that's my HP

Stage 2 comes around, and I have 10 VIT, so +2 bonus
I'd have 5 HP after that.

By stage 3, I'd have 20 VIT, so +4 bonus
I'd have 9 HP

With this system, however, investing in your VIT CAN get you both more HP now, and more HP in the long run... I just didn't demonstrate that for simplicity's sake.