Man, Sapphire have been using that same drawing for a long, long time. It appears on a sticker on the anti-static bag of a Sapphire HD 7850 that I have.
It's just them covering all bases without having to create something new for each card, which is why it lists so many different configurations. I wouldn't expect that connecting a 6-pin would work, but it's up to you if you want to try. You could try one of those 6-pin to 8-pin converters as suggested, if you're comfortable with it. Never been a fan of such things myself, but people must use them I suppose.
My worry about the PSU itself wouldn't be its specs necessarily, but just its age in general. Capacitors fail over time, and Vega is pretty power hungry. It's quite a load for an old PSU, especially when you need to start using adapters to even connect the card up. I wouldn't risk my shiny new card with such a setup, but maybe I'm overcautious.