Hi Bob, I'm assuming you're talking about Gooseneck type vents.
I'd suggest Mold Max 10 for ease of molding and good flexibility for pulling the parts out of the mold.
The way I do it is having the vents standing up (vertical) and mounted on a runner about 3/16" square. Make the mold about twice or 2 + 1/2 as wide as the runner. You want it thin enough you can squeeze the finished mold to compress the cavities that are the vents.

To make the parts, put a toothpick down the hole of the vents, pour the runner full, then pullout the tooth pick. The resin is then forced down the hole by air pressure against the vacuum caused by the toothpick being pulled out. Then pinch the mold at the bottom to force any remaining air out of the vent cavity.
I use the Smooth On 305 resin to give enough time to get it done.
Hope This helps, David @ Loose Cannon East