No one has gotten the general rule right yet. "Either" and "neither" take a singular verb in some cases and a plural verb in others, as follows:

1. We start with the general rule for compound subjects separated by "or":
If both subjects are singular, use the singular verb:
Harry Harmer or Jane Jones is coming to visit tomorrow.

If both subjects are plural, use the plural verb:
The Harmers or the Joneses are coming to visit tomorrow.

If one subject is singular and one is plural, the verb form agrees with the subject closest to the verb:
The Harmers or Jane Jones is coming to visit tomorrow **NOTE: CORRECT, BUT AWKWARD AND DISFAVORED**
Jane Jones or the Harmers are coming to visit tomorrow **PREFERRED**

2. It's the same if we add "Either" to the front of the sentence:
EIther Harry Harmer or Jane Jones is coming to visit tomorrow.
Either the Harmers or the Joneses are coming to visit tomorrow.
EIther the Harmers or Jane Jones is coming to visit tomorrow. **CORRECT BUT AWKWARD**
EIther Jane Jones or the Harmers are coming to visit tomorrow. **PREFERRED**

3. FInally, it's the same if we substitute "neither" and "nor" for "either" and "or":
Neither Harry Harmer nor Jane Jones is coming to visit tomorrow.
Neither the Harmers nor the Joneses are coming to visit tomorrow.
NeIther the Harmers nor Jane Jones is coming to visit tomorrow. **CORRECT BUT AWKWARD**
Neither Jane Jones nor the Harmers are coming to visit tomorrow. **PREFERRED**

Yes, in your example, you use the singular, because what you really means is "neither vodka nor rum is ideal." But if you replace vodka and rum with "doughnuts" and "cookies," then you would use "are."

Saying "both are not ideal" would be a bad mistake, because it is ambiguous. Do you mean "neither vodka nor rum is ideal"? Then say "neither one is ideal." Or do you mean "Using both vodka and rum together is not ideal"? In that case, say "using both together is not ideal."

As you can tell, I'm a lawyer. Who else would actually LIKE this stuff?

Cheers,
Mike