The zeta regularization of basic commutators in string theories recently pr
oposed by Hwang, Marnelius, and Saltsidis is generalized to string models w
ith non trivial vacuums. It is shown that the implementation of this regula
rization implies the cancellation of dangerous terms in the commutators bet
ween Virasoro, generators, which break the Jacobi identity. [S0556-2821(99)
04802-X].