If my trans temp is near the pan rail, how much fluid will I lose if I removed it ? is the pan full when it’s off?

None if you lay car on its side haha so if you pull dipstick out and you may be able to reference its level that way are you trying to replace sensor if it was me I would loosen drain plug and let some fluid drain just to be sure

Wonder if the vacuum trick would work in this instance

Reason I ask is I’m gonna have to pull trans temp sensor. Was hoping I would mot have to drain

Jack up the side of the car the sensor is on, if it's not over filled you're fine.

Use a finger over the hole, might lose a little just have to be fast. We would do oil pan drain plugs like this if the seal was leaking. Just do it cold.