I have spent lot of time doing research on VIM. I am Windows guy since last 6 yrs and was using VS. Now started working on Linux. I want to make VIM as close as possible to
These plugins used to exist long before vim had tabs. I'd be quite surprised there isn't a way to tune these plugins to split windows instead of opening tabs. Now I can't help you much as I don't use these specific plugins but other ones. You should look at their help (:h project, :h taglist, etc)
PS: in vim terminology (it will help you browse the help files), what you call "buffer" is actually called "window", while a "buffer" is just the text you are working on, it may be associated to a file, or not. For a given buffer, there may be no or several window displaying parts of the buffer.