Don't think that it is a 'vain' thing to worry about the acne. You are human and want to look the best you can. Very different to being vein.
I'd honestly speak to your doctor about this; we can give our opinions, or what the internet has provided, but your doc will be able to really understand the side effects in the way necessary to tackle this issue.
You didn't mention it, but I'm assuming the the lithium worked for the prescribed purpose and it is just the side effects you worry about?

Hope things are more clear now compared to when you posted.
Regards,
Tic Tac