You don't have a vacu-flush, you probably have a jabsco electric
You push a button to run a macerator to pump the waste out.
If you pushed a pedal and heard a giant sucking sound, you'd have a vacu-flush.
The problem is likely that you need a siphon break on your raw water
intake to the toilet. I'm guessing you flush, and the toilet drains, and then you let it sit, it slowly fills up... too much.
If you put a siphon break on the water line you may need to put a solenoid on it so when you flush it closes the siphon break.
Since you're new to boating
, you should also know that when urine mixes with salt water it will create mineral deposits that will coat the inside of your hoses, so every year or 3 you will need to take your hoses off and clean them out or replace them. Otherwise you can maintain them by soaking vinegar in the system to get rid of the deposits, and using lots of water to flush the urine from the hoses.