I moved all life to a spare tank, drained the main DT and ran it with tapwater for a week.
The rest is what I did out of order, just a list of my methods;
Plugged the drains and filled the overflow and plumbing with straight vinegar, then let soak for maybe 48 hours - pew.
Soaked all hardware in vinegar in buckets; thoroughly scrubbed, rinsed, then dried for over a week. Poured used vinegar into the sump, 1 chamber at a time.
Each chamber was pumped into the next to cut down on costs.
I went through 11G of vinegar in total.
I may have missed something ...
I then put the system back together and ran it for a week with tapwater
New LR, new sand, all SPS FRESH CUT frags heavily dipped and inspected with a jewelers glass.
7 Months Vermetid-free so far
EDIT: that was 11 Gallons of vinegar used, not 11 litres