Good afternoon,
My SIL had a VSG, she didn't lose any hair at all. Her locs were baby locs less than 2 years old.
My locs were about 3 inches past my bra strap.
My locs started to thin a little at the root, so I changed from twisting them to interlocking.
That made a little bit of a difference, I chose to cut my locs, now they are shoulder length.
My thinning was hereditary and not caused my weight loss.
That said, I keep up with my water and my protein to keep the hair loss to a minimum.
Final answer, no you probably won't have to cut your locs unless you want to.