"Neither" can be singular or plural, depending on what it refers to. ("Neither the teachers nor the administration are being reasonable in these negotiations"; "neither the teacher nor the dean is being reasonable in these negotiations.") "Vodka" and "rum" are singular; hence the choice is one singular thing or another; hence the "is." -- BTW, often, when in doubt, one can take the coward's way out and say "neither would be ideal." No agreement to deal with.