From my experience extending threshold over an hour, and therefore VAM for 1-3 hour HC climbs, can be done by doing a) twice a week muscular endurance intervals, b) some short but focused above threshold work, and c) once a month doing a really long 1-3 hour session. Do that over three or four months. Roughly the same timeframe as SSB 1 and SSB 2. At my age and ability to recover, that seems like a better return on training investment as it leaves some energy budget for strength training and time for recovery/adapations.