Too many Vaults is a rare problem, I think, as you can always discard vaults to vaults, and the vault deck doesn't hit $5 that often anyway (and $6-$7= gold, naturally, even with Tunnel around)
As for play rules for Vault-Tunnel, I'd suggest:
Silver till Vault acquired.
Then:
First $3-4, buy one tunnel, if not on $3-4 follow rules as below:
Then:
$8 = province, unless PPR applies or lose game on buy.
$5 = Vault, unless behind on VP or <5 provinces remaining, in which case duchy, or if have at least two vaults and four golds in deck, in which case duchy.
$6 = Gold, unless four or more golds in deck, in which case duchy
$3/4 = Silver, until silver > tunnel +4, then alternate tunnel/silver
$2 = estate if estimate less than two shuffles before end of game, otherwise, pass.
$1 = scream and rant a little, and end turn.
For Cartographer/Tunnel I'd suggest silver till Cartographer, then:
spend $3-4 on Tunnels until TWO tunnels, after that by silvers unless the VP from the Tunnels will affect victory outcome
$8 = province, unless PPR applies or lose game on buy.
$6 = Gold, unless four or more golds in deck, in which case duchy
$5 = cartographer, unless time for duchies
$3/4 = Silver, until silver > tunnel +4, then alternate tunnel/silver
$2 = estate if estimate less than two shuffles before end of game, otherwise, pass.
$1 = scream and rant a little, and end turn.
My instincts suggest those would be the optimal approaches, but I could be wrong. Tunnel is complicated, of course, because of its VP value, and there'll be turn by turn assessments as to whether it helps or not.
As a rule of thumb, I'd say that only buy more than one or two Tunnels when the VP is worth something to you. I think people often get too interested in the gold generation effect and over-commit to Tunnels. Unless there's Silk Roads or I'm nudging along a favourable three pile ending, I rarely want more than two tunnels. In particular, I think Vault/Tunnel loses tempo if you see the tunnel as anything more than a route to setting up the Vault/Gold engine, and indeed too many golds will stop you drawing Vaults, and might leave you on annoying 6 or 7 dollar hands, rather than the optimal Vault + Gold + three cards. I reckon that if the simulator generates more than $8 on a province buy hand on more than half the buys its buying provinces, its probably being inefficient.