My 12mm vectran outhaul has a polyester cover and it has not slipped in 3000+ miles and winds in excess of 50knots.
Just pull it up as tight as youcan and it will be fine.
The 12mm fits much better in the jaws of the Anderson line tender than 10.
You may find Vectran without the polyester cover to be more slippery than with the polyester cover.
SV Elyse SM437
Noumea, New Caledonia