>>Is it possible to completely secure the content of a sheet while still allowing VB to use it?
In a word, no.
And BTW, you can also write a formula in Excel that refers to a 'very hidden' worksheet. If the end user knows the worksheet exists, even if s/he cannot see it, s/he can still find a way to get to the values.
You can try using some sort of encryption on those values, but then your VBA would have to encrypt/decrypt the values, and if anyone can get to the VBA code, then your encryption is compromised. Don't even say "I can lock the VBA project"--that is very easily hacked.