depends on the vape … specifically if the herb is inside the heater, or in contact with the heater, then some will be wasted - stopping and restarting will be disappointing. if the herb is mostly outside the heater, then hot air is drawn through the herb, it is instant on-demand, so nothing is wasted … so this type you could take a hit, walk away and come back, okay.
in my vape i find most of the cannabinoids are extracted in the first 2 or 3 hits, so coming back to continue the session is a non-starter for me.