7.5 cast/sec
4.0 trigger/sec desecrate
4.0 trigger/sec offering
15 corpses generated
9 corpses consumed per cast of VD
4 cap on offering
67.5 vd/sec 16 offering/sec

.133 .266 .4 .533 .666 .8 and .933 (1.066 excluded) are the 8 cast times, rounded down to 7 in a window of 1 second.

.25 .5 .75 1 are the 4 trigger times of cwdt in a 1 second window.

Order is cast begins, trigger starts, next trigger cannot take place until after 250ms has passed. 2nd cast falls at 266ms, which will consume the corpses generated, and trigger cwdt again. 3rd cast can eat remaining corpses on the ground. 4th cast comes in at 533ms and can trigger again. 5th cast comes in at 666ms and eats into the remainder even more heavily than the 3rd. 6th cast triggers cwdt. 7th eats into remainder even more heavily than the 5th.

0ms: 15 corpses generated. If order of gems is suitable following global trigger order on cwdt wiki page, then 4 will be consumed by offering in this same instant. Reduce total to 11.
133ms: Cast begins, consumes 3. Total down to 8
250ms: CD is restored, but corpses not yet generated.
266ms: consume 3+4, generate 15. Worst case scenario depending on trigger order, back down to 8.
400ms: Cast begins, consumes 3. Total down to 5.
500ms: CD is restored, but corpses not yet generated.
533ms: Cast begins, potential dip to 0, but still sufficient to cast both spells. (VD may lose 1 ball in worst case scenario.) Could go up to 15, or back to 8. Assume 8 for safety.
666ms: Cast begins, consumes 3. Total down to 5.
750ms: CD is restored, but corpses not yet generated.
800ms: Same case as before, assume back to 8 for safety and that 1 ball may have been lost.
933ms: Cast begins, consumes 3. Total down to 5.