My mechanic says my serpentine belt and tensioner need to be replaced and quoted me $400 for the work. I have a 2004 Pontiac Vibe that might be nearing it's retirement time and am skeptical about putting this much money into a simple belt replacement before it is absolutely necessary. How critical is it to have this replaced when a mechanic identifies it as a problem area, and is $400 a reasonable quote?
That is way too much money for a serpentine belt and a tensioner pulley on any vehicle. If you cannot do this yourself, take it to a different mechanic and have them do it. A serpentine belt is not at all critical to the overall health of the engine. If it fails, you put a new one on. It may be inconvenient after it breaks, but it's not going to ruin anything in the mean time. It's inconvenient because your alternator, power steering, and most importantly, air conditioning will no longer function. The engine, however, will continue to run.
NOTE: While no major evil effects will happen, it is important that you wouldn't run your engine too long as it is at this point, only running off of battery power. You will eventually drain all the juice from it and the car will die. Once the new belt is put on and the battery recharged, everything should be back to normal.
EDIT: @yaymuffins pointed out that the serpentine belt also runs the water pump in this application (the Toyota 1zz-fe engine). If the serpentine belt completely fails, this becomes a major concern. Serpentine belts do not usually fail so spectacularly, but it is a concern. If the serpentine belt fails completely (you'll see a lot of dash lights come on), you'll want to shut your engine down rather quickly to avoid over heating damage.
Now, with all that said, are you sure they said serpentine belt? They surely could have said timing belt, which is very important in the grand scheme of things. If it were to fail, it could destroy the upper end of your engine. Please take this into account when you are making your decision as to whether you are getting the work done or not.
Regarding your question of how critical the belt may be, it is very critical. The 4-cyl Vibes run the water pump with the serpentine belt. If the water pump is not running it can overheat your car very quickly.
The $400 price seems steep for your vehicle, I recommend using the NAPA auto repair estimator to give you an idea before talking to a mechanic. http://www.napaautocare.com/estimator.aspx
If you are at all mechanically inclined and have tools , I would recommend giving this repair a try on your own. There are plenty of how-to videos, here is a link to one so you can decide if you want to give it a shot. https://www.youtube.com/watch?v=IDPFq-9jnXE
Just do it yourself. It is a beginer's job. Parts only costs less than $100. Watch Youtube to learn. The belt is quite important as it powers everything from alternator to powersteering and could leave you stranded. However nowadays most cars have automatic tensioner, which is a bit like point and shoot camera, not much can go wrong.