Vaping, unfortunately, uses much more weed than smoking (you're not using 100% of the mass when you're just melting the crystals), so there's no good answer for that. I do recycle what I vape from (merely by smoking it conventionally - takes more, but it's still potent
).
Slight contradiction between the "uses more" and "use what's left" part. Vaping is more efficient--you're not using all of it. What is left over isn't used up, like you said. You can always revape it again and again, and usually that will use almost all of it. It's not so much that it physically uses less so much as you have to re-vape it to extract it all. One pass isn't usually enough on most vapes. I know I stir mine in between draws, and that helps. But in the end, you are using it more efficiently than burning it, which is a lossy process.
Plus, whatever is left over even after only a single vaping can still be smoked or made into edibles. It seems like it would be less potent to smoke afterwards, but I find it works fine, if slightly decreased. Actually, I tend to get super strong body highs off of smoking vaped weed compared to regular cured bud. The smoke is harsher, but nice for sleeping, imo.