Hello from Washington,
This is just my two cents. I work with 4160v through 34.5kv in underground situations every day. because I work in the mining field I fall under the 30 cfr instead of the 29 cfr, but we do adhere to the 70E standards, so take what I have to say for what it is worth.
First thing, I would set up an emergency retrieval tripod with a winch rated for twice the wieght of the largest man. I would put a harness on anyone heading into the vault so I could pull them out in an emergency by my self.
I would set up ventilation on site and have fresh air blowing into the vault for at least 10 min before entering. I would drop a gas meter into the vault to make sure there were no deadly gases left in the vault where I was sending myself or my men.
Next I would have my men in the appropriate PPE for the voltage present, lower them in, cover the exposed lines with blast blankets, remove the flash suit, and put the retieval harness back on.
After finnishing the job required I would have them put the flash suit back on, remove the blanket, clean the vault, and hoist them back out. Then clean the site, reinstall the vault cover, and then test the new circuit.
I know my approach may be a bit lengthy and time consuming, but a life is never worth less than money.
You might want to even sit down for five min and do a pretask plan to make sure everyone is in the same frame of mind and noone has missed any of the site specific hazards.