2003 Ford Windstar Vacuum Leaks

How hard is it to repair a vacuum leak and if left unrepaired can it damage another part of the engine.
My van was running really well after I took it to the dealership to have the check engine light checked out. At that time he told me I need to replace the spark plugs and wires, and that I had a vacuum leak. When I got it back it ran like new, I don't know what he did do other than replace 2 of the spark plugs that he couldn't clean off and told me to have my son-in-law (who is a mechanic) to replace the other 4 and to replace the spark plug wires. We did that and everything was okay until my son in law decided I need to power spray the engine compartment clean. The minute he did that the engine ran rough and started backfiring again, idles rough and sometimes will stick in the middle of shifting when I speed up during normal driving conditions. This has gone on for more than 6 months, I don't have the extra money to take it back to the dealership and have them repair and I am worried to allow my soninlaw to touch it again. What is going on with this car?