Since each manufacturer uses a proprietary mix of silicones, there is no general rule. You'll need to rely on manufacturer's data. Some companies do give actual vapor pressure at specific temperatures, e.g.,Silicone 704: "Vapour pressure @ 25°C 1x10-9 mbar".
Why not contact the manufacturer directly, instead of asking a third party?
But my source of confusion is that ambient pressure is not the only pressure pressing down on the liquid.
Pressure does not press down. When something is under pressure, it exerts it in all directions. Maybe it would help imagining this problem in the absence of gravity (you would have to put the sample in a stretchy balloon to get some pressure while being ...