Plain vectran (like vectrus) is relatively cheap but you can't hold onto it. Tapering is the way to go (figure you know what this is.. but it is stripping the cover off the core and burying it so it has a nice taper and runs through blocks perfectly. you end up with a smaller diameter, slippery high tech core going through the blocks).
Lines like Marlow V2 (may be discontinued) have a vectran core which can be tapered. For some reason, the taper doesn't look as nice with the vectran core as it does with a dyneems core (marlow D2- which I think may have been discontinued as well). I did a taper on the same diameter V2 and D2 and the D2 lookes much better... BUT... not talking about looks.
I think the problem with the salsa/conceptions stuff is that it can't be tapered (maybe I am wrong here, but it is listed as a single braid) so you are stuck with the whole diameter running through everything.
5/16 I think is the ideal size and shouldn't be a problem with a 10:1 purchase system.
You could be the first on the block with the new Paraloc line (check out aps.com) which looks interesting but isn't taperable either.
I would go with 5/16 D2 (if you can find it still) with a taper. Can't help with the length though.. sorry, but on my boat with 9:1 blocks and using the tail as the trav line, I use 45 feet and it seems a little long. I would start with 50-55 (???) for your boat and decrease from there.
If you can't find the D2, warpspeed would be perfect and is very easily tapered.
One thing that I think I will try in the future is use some vectran but either have someone (aps, trey, etc) put a cover ON it or do it myself.
hope I didn't confuse you or anything.. lines can get complicated.