Are you talking about a low level alt capture so the pitch mode changes to ALT ACQ and the thrust is reduced? If so just bug up to min clean speed. Otherwise leave it in TOGA or MCP SPD (if autopilot is engaged).
At high alts I always use level change (= Mach hold) - I would not use or condone the use of V/S at high altitudes even with the inbuilt protection provided.
Hope this helps.