Probably the most practical way of digitising VHS tapes would be to sample the composite video signal at 4fsc (i.e. 4 times the subcarrier frequency). The professional D-2
video tape formats store a 4fsc-sampled composite video signal. There's probably a corresponding SMPTE standard for digital transport of 4fsc-sampled video. That should capture all data in the vertical blanking period.
Unfortunately I don't know of any hardware (consumer or professional) which can sample a composite video signal at 4fsc into a computer. Typical video capture hardware decodes the composite signal into component (YPbPr) first, then samples luma at 13.5MHz, chroma at half that rate. For preservation purposes, not decoding the composite signal before sampling would be better in my opinion.
Some professional (S)VHS decks might be able to output separated luma and chroma when playing VHS tapes. If you could sample the luma at 4fsc (and chroma at 2fsc maybe), that could give a better quality capture.