There are a couple of things that can go wrong. The first one is leaving marks on inside portion of glass. Once you seal glass, you cannot clean what's between panes. The other thing involves condensation between panes. If you have even slightest break in silicone seal around glass, chances are you will begin to see moisture form as soon as nights get cold and days get warm. You are going to have to decide if you are confident enough in your ability to do job right, or if it's better to pay extra money to have it done for you. Just because you pay someone to do it, doesn't mean you still won't encounter same problems. The difference is, they have to guarantee their IGU for a minimum of 1 year. I have received many units over years that had marks in between glass. The beauty of it is manufacturer can't dispute it, because there's no way anyone else could have done it except them.

OK, what if window frames are vinyl instead of aluminum? Well, main difference is glass in a vinyl window no longer has rubber gasket around edge. You don't remove opposite corner screws and separate frame from glass. What they do is put either silicone or a two sided tape on lip of frame where glass rests. That's what holds glass in frame, then they apply a snap in stop on all four sides of glass. So, you have to remove stops first, then turn over panel and break seal holding glass to frame using a utility knife. Wear gloves during this procedure. If only one side of IGU is broken, don't even think about repairing just one side. You will never get that IGU out of frame without breaking other piece of glass in process. But, on positive side, you can remove stops without taking panel out if it's a slider. You can then measure dimensions of glass, and order new IGU. That way you eliminate any need to temporarily cover up your window. The same is true for stationary portion of a slider, or a picture window. Before you install new IGU, be sure and clean lip that had tape or silicone, and apply either silicone or tape. Either will work.

You will discover that replacing an IGU in an aluminum frame window is a whole lot easier than a vinyl window. But, in either case, you can do it yourself and save a few bucks.