Is it possible to insert 50 Kb of DNA to a bacterial vector and successfully maintain and amplify it in bacteria? If so, what would be the best vector?
That would be difficult to answer. I guess the three options are a phage (generating a non-infective prophage to maintain your foreign DNA in the bacterial genome), a cosmid (where, however, 50 kb might be too large for the prep and maintanance procedure) or a BAC (which is an artificial chromosome, normally best suited for even larger inserts). For the three options, numerous vectors would be available.
In the market there are many types of vectors and depending on what you want to do with the inserted fragment is the vector that you will choose. But if what you want is to just clone that is stable and I recommend a Fosmid.