What happens if my shares aren’t vested on the 1-year cliff and we are acquired?

Yup. If there is no acceleration clause and the company is acquired before your 1 year cliff, you could be out of luck. Many companies try to do the right thing and accelerate a portion of it even though they weren't obligated to but not always.

In the absence of a cliff it depends on the vesting schedule. Assuming a monthly vesting schedule over a four year period, you'd be legally entitled to 1/48th of your total shares every month that you're there.