I looked up your Vektron and the Tern site says the grips are Ergon GP10 which I don't even find at the Ergon website. Could that be a typo and they are GP1 grips? I'm having a similar issue with my Tern Eclipse S18 (non electric). It has GP3 grips which have the bullhorn things. I might have to change them to GP1 grips in order to attach a bar end mirror. I need the mirror more than I need the bullhorn grips.
If you have the GP1 grips, I think any bar end attaching mirror will work. The Busch and Muller mirror that I posted in the thread above is attached to my Riese and Muller which has Ergon GP1 grips. The mirror folds away nicely (appropriate for a folding bike) and offers a wide field of view. Its the better of the two mirrors I currently use (a Mirrcycle on one bike and the Busch and Muller on the ebike). With a bar end mirror like the B&M you might just have to test out the various fold positions to see what works the best when you fold up the bike (to avoid damage to the mirror). Not sure what I'm going to put on my Tern yet but I might go the route of switching to the GP1 grips and attaching the same B&M mirror (even though it is sold as an ebike mirror I don't see why it wouldn't work on my Tern). Let us know what you choose. I have 3 Terns in my household by the way (all non electric) - 2 that I ride and 1 is my wife's.