Great to know that you are planning to get into investment banking domain. It is one of the most sought after career in finance field after private equity. and as it is one of those big deal stuff, competition is equally intense. Best and brightest from top B-schools try to get into all these bulge bracket IBs but note that it is a very top dominated market as it pays to be in those big ones but not if you are working in some small broking house as then it will be shade of what those guys do.
Hence part time MBA or executive MBA will not be able to give you required shift and best bet will be one year MBA program from any top rung schools. Hence you need to plan about it.
hope i have not disappointed u