Hey ya MP,
Not entirely sure of the degree of cross-tolerance between the two but I would bet that the MS would pick up nicely where the Tram left off. Of course you might have to bring a little bit of an antihistamine on board to ease some of the histamine issues with the MS but other than that it should be a big help. The Venlafaxine might have a little lag time catching up but should stop any SNRI WD in it's tracks. Although MS & Venlafaxine are most likely going to be a bigger beast than the Tramadol was.
If your goal is total abstinence then I would be cautious of the length of time you go down this road as Venlafaxine WD's are said to be worse than Tram's for some. If you are using IR Ven. then I think it's 3x/ day dosing equaling 75mg total but it may not require this much to ease off of the Tramadol vs combating depression. On the conversion of MS to Tram I think it's 30 of MS is = to 150 of Tram, so it shouldn't take very much for WD mitigation. For just a short period seems solid, just be mindful of the usual Serotonin Syndrome risks if other meds are in use or will be. Hope it works out.