When exploring the north the Ha Giang Loop is by far the most popular one, but Cao Bang with Ban Gioc waterfall and Pac Bo cave becomes a more interesting destination for motorbike trips. With enough time both the Ha Giang Loop and Cao Bang Loop could be combined for a great trip to the far north of Vietnam.
Coming from the Ha Giang province the best option is to head towards Cao Bang from Meo Vac, as it is closest to the other province, and the route covers some nice scenery. Adding the route to a north Vietnam trip would be worthwhile.
I would suggest taking two days for the approximate 200km, with an overnight stay in Bao Lac (or as alternative in Tinh Tuc). From Cao Bang the same route can be used to head to Meo Vac – though an alternative route will be highlighted as well.