One of the issues is not knowing if the bulb is genuine or not. If it isn't you aren't going to get the same brightness and lamp life you would from an OEM genuine bulb. If you are replacing just the bulb you need to wear gloves, because if any oil from your skin gets on the bulb it will shatter once the bulb is running and reaches a hot enough temperature. You also need to be careful about making sure the bulb is seating seated properly once you've replaced as that can cause issues as well. If you've never done it before just take your time and document exactly how it looks before you start so you know you replaced it correctly.
If you end up not feeling comfortable enough, I know AVScience has the best price on genuine OEM replacement lamps (with a new housing) as I've bought from them in the past for my VP-11S1 and VP-15S1. The 11S2 takes the same bulb. I searched high and low and AVScience is one of very few carrying genuine lamps for these projectors still and they have a better price than some non-OEM ones.
Here is a link to AVScience's 11S2 bulb: