got new plugs installed, still a semi-rough idle
Then the plugs weren’t the problem or at least not the whole problem. What prompted you to have new plugs replaced? Is the check engine light on? Assuming the air filter is clean, a bad oxygen sensor is one possibility, so is a dirty throttle body.
Maybe vacuum leak too.
Entirely possible. Is the check engine light on? Regardless, take it to a good independent mechanic for a proper diagnosis and repair. If a shop recommended the new plugs give them one more chance to get it right.
Just my 2 cents, but one should always rule out a mechanical fault before replacing parts. This means a compression test. In cases where the plugs are difficult to access a pre-test could be done with a vacuum gauge. If the vacuum gauge shows an issue then it’s time to pull the plugs and run the compression check.
You might elaborate on whether or not the CEL is on, who changed the plugs, and the reason for doing so unless it was just guessing and guessing can become time consuming and expensive.
Is this an engine that needs valve adjustments?
Yeah, good point!
Vacuum gauge May jump and not read steady if there is a mechanical issue.