Two questions:
1) what do actually mean with "low-magic" campaign, do you mean that there spellcasting is rare or that magic items are rare? The two things are quite different in practice, I have no idea how Grim Tales handle this. I assume you just mean that healing is not going to be so readily and often available
2) are you sure that VP/WP works better when there are less healing spells and potions around? I think UA effectively suggests so, but I wouldn't be completely sure... to me it has always sounded as a higher risk of sudden death exchanged for faster recovery, which IMHO is inconvenient

. Furthermore it is quite a complicated variant...
Since you have Unearthed Arcana, one of the suggested variant for a low-healing campaign is "Damage Conversion", which basically mean that any armor converts part of the damage into nonlethal (amount = armor bonus). The consequences are that an armored character is more rarely killed in combat unless CDG-ed, and that the PC will heal at a faster rate than normal.
I also think you can use a sort of turbo version by combining the Damage Conversion variant with the Armor as DR variant, which by itself doesn't make combat less deadly, but together with Dmg Conversion can likely turn almost ALL damage into nonlethal.
Otherwise the "Reserve Points" from UA is pretty simple as well and it can be either used alone or "on top" of the previous variants.
Finally, if you just use the Defense Bonus, you won't get any benefit. Battles may last longer, money can be spent on something else than armor, touch spells will be less likely to hit, but I cannot see how the variant would help with the lack of healing.