is there a market for customizing web apps like SalesForce, perhaps in competition with the vendor?

Factual Questions
1

I seem to have read that some web apps out there have several levels of licensing where you get more features for higher level license. Then if you have lots of employees you probably pay a lot for those extra features for every one.

Is there a market for providing the equivalent features via a separate web app that would integrate with the original and rent access for less, hence milking the vendor’s business model? Are there any features that you might really like to have in your web app that but were afraid to ask the boss for the outlay of licensing them from the vendor?

Oops, clarification. So imagine that the customizing web app could be built as a mashup, without needing to access the code base on the vendor’s server. Kind of like adding a few features to Gmail gui via javascript, an extra server and various other tricks, for example.

2

There might well be a market; that is, willing buyers at a profitable price to you.

Whether the base vendor’s TOS would permit what you what to do is another matter. And only by reading the particular TOS of the particular vendor will you be able to answer that.

Some vendors like the idea of an ecosystem of value-added 3rd party accessories. Others want all the cookies for themselves.