But then I got curious & started looking up the quote, wondering about the context.
It turns out that the quote is spoken by the foolish, arrogant Dauphin, so I'm not sure Shakespeare intended it to sound like good advice. On the other hand, the King of France (to whom the advice is addressed) is in a position to represent his country in a conflict, and so thinking highly of himself (self-love) would almost certainly be better than presenting himself as uncertain of his own worth (self-neglect). He wouldn't want to make himself look weak ... though I don't think he would want to give unnecessary prideful offense, as the Dauphin does earlier in the play, either. So I'm not sure exactly what Shakespeare's intention was in having these words spoken by the Dauphin.
If we assume that Shakespeare intended the Dauphin's advice to be good—that it is better for the king to value himself too much rather than too value himself too little—then it would nonetheless have to be balanced against Henry's frequently modest, self-effacing attitude. (Note: I totally want to refer to him as "Harry," but I think that's probably inappropriate for this play.) Henry (the hero of the piece, obviously) shows tremendous "self-love" if you define his "self" as his country, but shows very little "self-love" if you define his "self" as his own individual person. Much is made of his willingness to identify himself with the common man, and, in fact, the word "vile" is used again in another famous quote on this subject:
We few, we happy few, we band of brothers;
For he to-day that sheds his blood with me
Shall be my brother; be he ne'er so vile,
This day shall gentle his condition
Henry is telling his men that he does not hold himself as high-and-mighty, lofty and above them, that they are all brothers in this, and there is no vanity in that speech, but there is pride.
I think the Dauphin's quote is basically saying that vanity is better than low self-esteem, and I certainly would never say that Henry had low self-esteem ... but neither would I say that he was vain. But he does certainly think highly of himself, his cause, and his army. (Otherwise, nothing in the play would ever have happened.) Perhaps, in fact, despite his modesty in many things, Henry V would agree with the Dauphin's statement: a king must present a strong front and would, most likely, serve his people best by thinking too well of himself vs. thinking too poorly, if those were his only two choices. After all, both ends of the spectrum are being presented as sins—vanity is not being presented as a good thing—the sin of "self-love" is just being described as the preferably one, if you're forced to choose between the two extremes. And I guess I could get behind that, though I've lived my life by the exact opposite maxim up until recently (or, realistically, even now, though I'm working on it).
Still, having the Dauphin be the one to express this opinion makes its intended wisdom questionable, in my opinion. Unfortunately, I don't have Shakespeare's phone number, so I can't just call him up and ask him what he intended.