"We can’t tell whether he did 'this' or 'that', or if he was on vacation."
As far as I know, a "whether ... or" clause can typically be replaced by an "if"-clause, which is often just a matter of style. Now, considering the example sentence above, would it be okay to write it like this? The first clause would allow me to distinguish between 2 options ("this" and "that"), which is why I'd prefer "whether ... or" over "if."
Alternatively, would it be better to rewrite such a sentence, for example:
"We can’t tell whether he did 'this' or 'that', or whether he was on vacation or not."