The reptile vets I've had (all three of them) also recommend soaking for a snake with an RI, if it shows any signs of dehydration. Following their advice, I haven't lost a one. The snakes always appear to feel better afterward.

I also absorbed a passing comment that the positioning of an injection isn't actually as important as they used to think it was--in other words, injecting into the back half of the snake isn't seriously riskier than injecting into the front half.
I do not think the vet is to blame for this loss.