I have a solution for this - as I've been looking for months.
Thankfully, the latest (at time of posting) MSI Afterburner, version 2.2.0 now does this! You can enable it under the "monitoring" tab in "Settings".
I've tested it on my Radeon HD 5850 1Gb and it's working correctly, as Skyrim with high texture pack mods runs 60fps when vram showing 888Mb and drops to 16fps as soon as I look around and vram usage shows 1134Mb.
Right - I'm off to tell all the other forums I've been checking for months!