EDIT: I think the confusion here is that your first source and perhaps also your second are using the abbreviation to mean something different from what the third one is using it for.
If you look at the abbreviations in the first, and what they are used for, the v.n. abbreviation is always attached to intransitive verbs. It turns out intransitive verbs were once referred to as neuter verbs, as shown here from Webster’s Revised Unabridged Dictionary (1913):
(b) Intransitive; as, a neuter verb.
The v.a. appears to be attached to transitive verbs. It is possible that these were called active verbs because, since they took a direct object, they could be formed into a passive. It is also possible that they are contrasting them with stative verbs. The same source as referenced above gives three possibilities:
(a) Applied to a form of the verb; -- opposed to
passive. See Active voice, under Voice.
(b) Applied to verbs which assert that the subject acts
upon or affects something else; transitive.
(c) Applied to all verbs that express action as distinct
from mere existence or state.
That source also uses v.s., which may be a stative verb, at least in the case of beis, which it says is the 3s form of to be. However, it also uses it for things like bele to burn, conteyne to continue, etc. So that may not be right.
It also has v. imp., v. impers., v. aux., and v. subst.. Considering it seems to be using v. subst. are verbs that mean “are”, the v.s. as stative is probably wrong. Besides v.n. and v.a., it has these:
- AFFERIS, EFFEIRS, v. impers.
- ARN, v. subst. Are, the third pers. plural;
- AUCHT, (gutt.) v. imp. Ought, should.
- To BECK, BEK, v. s.
- To BEIR, BERE, v. s. To roar, to make
- BEIS, v. s. Be, is; third p. sing. subj. S.
- To BELE, v. s. "To burn, to blaze."
- BENE, v. subst. Are.
- BYRD, v. imp. It behoved, it became.
- BOOST, v. imp. Behoved, was under
- BUT, v. imp. Expressive of necessity, S.
of another, S. from the E. v. catch.
- To CONTEYNE, v. s. To continue.
- DOID, v. imp. It becomes, Fr. doit.
- GURDEN, v. 3 pl. Gird.
- ILD, v. imp. Would not.
- LYK, LIK, v. impers. Lyk til us, be agreeable
- MEDIS, v. impers. Avails.
- METHINK, v. impers. Methinks.
- MIRKLES, v. pl. The radical leaves of
- MOT, v. aux. May.
- MUN, v. aux. Must.
- TID, TYD, v. impers. Happened.
- WALD, v. aux.
- WAR, v. imp. War him, befal him.
- WORDIS, v. imp. It wordis, it behoves,
- It worthis, v. imp. It becomes.
The third source is clearly using the abbreviation in a different way, one that means “meaning” or “roughly” or “for”. I do not think it stands for “version alternate” there, but I have been wrong before and doubtless shall soon be again.
Original posting. While v.a. does mean this, it at most applies to the third source only.
The abbreviation v.a. stands for the Latin vices agens, meaning “acting in place of”. That is, quite literally “vice agent”. Used in this way it should be read as for or meaning in English.
There is also v.a.l., which is vices agens legati, but that would not be used here.
Adolf Berger’s Encyclopedic Dictionary of Roman Law (1953) covers this and much more.