What tarbubble said about the residue. Esbit is messy. LNT. A bit of foil weighs almost nothing and it may reflect some extra heat toward your pot. Also, placing the tab on the ground may make it too far from the pot, depending on the design of your cone.

Regarding the vest, I've tried that idea and I still sometimes use a vest for dayhikes. The good: Easy access to your stuff. The bad: Bending over forward is more difficult. If you're crawling or climbing on rock the pockets scrape. Can be uncomfortable on a hot day.
If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.