portamento in general works perfectly OK with Dorico. I find the best way seems to be to create a new custom playing technique called "portamento" (which makes it easier to distinguish from those built in to the software) and add it to the Expression Map. You then also put a port slide in the score but you must disable the playback, otherwise it will override the VSL switch. In the score then hide one or the other as preferred.
It may be that John's suggestion also works but one other often critical step is to assign a "playback start offset" in the order of -10 to the note in question. This ensures that the port starts on the correct note. Several people on the Dorico forum have confirmed my suggestion works when they had otherwise had some problems.
A final point is that if you use the VSL SE template, you not only can but probably should be editing it to match your own musical preferences Simply adding a line assigning a keyswitch to portamento won't break anything -- if you make a mistake, just modify or even delete the line in question and nothing will get broken. Of course it's always best to export the original EM as a back-up just to be sure.