Can my ISP see my data usage if i use a VPN?

[Adrian]

All traffic, whether direct or via VPN, has to go through your ISP. Every bit and byte is counted, they don't care how you use it, just how much you use.
With a VPN, you may actually use a bit more, due to the security encryption that VPN uses (in general it will still be the same amount of traffic)
With VPN, the ISP cannot see what the traffic is (encrypted), but they can still see all the bytes that you transfer.