The imperfect subjunctive doesn't make much sense. He's giving a command (go with God) and the imperfect subjunctive would mean something more akin to (were they to go/have gone with God) which expresses a retroactive wish.
The indicative present doesn't make sense either: van con Dios states that they are going with God presently. That sounds like the foundation or conclusion for some other set of statements.
The present subjunctive is used to express a wish: may you go with God. In Modern Spanish, this wishful use of the subjunctive (called the exhortative subjunctive) was employed by the (then new) usted(es) pronoun to give it a command. So while it is technically the present subjunctive form, we would today generally consider to be the affirmative ustedes command, relegating the exhortative sense to things that are both grammatically and semantically third person.
In Latin America, the vosotros form is not used, as a result, id (the vosotros command) would be inappropriate for speaking to a Venezuelan audience. And since he was speaking to the Venezuelan people, the singular forms ve and vaya wouldn't make much sense, unless he prefaced it somehow with something like "y a cada persona de Venezuela, quiero decir «ve con dios»" or similar where the singular is logical.
