Or will US users be able to connect via a VPN?
Probably not a good idea. Slipping up once in a while or accessing from devices without VPNs could be a serious problem. Plus if you comply with KYC laws (
which you have to if you want to deal with fiat when Bitfinex starts handling it again), they will know you're from.
Where are current US users going to head as an alternative?
GDAX offers margin of up to three times on BTC trading as long as it's under $10,000. Right now I can't think of an exchange with high leverage for high amounts.