Q.I have a Mandevilla vine in the ground that does well for a while, then starts getting leggy and isn’t growing. can I take
it out and put it in a container, and if so, when is the best time? I’ve had it for a long time; the location is good and I don’t know if it’s geting too much water when the sprinklers go on. I don’t know how to take a photo and send it. Can I get an answer without a photo?

Certified GKH Gardening Expert
Pruning should help, but if it is being over watered and you can't change the irrigation, then you may be right about relocating or potting. In your SoCal location you can do that now this month or wait until spring, March/April. You could even cut it to the ground to rejuvenate it before new spring growth begins.
Here is info on pruning:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/prune-mandevilla-vines.htm