That depends on whether you interpret "when" + past tense in English to also assert the reality of the temporal clause. The interpretation which allows the vacuous truth is, in my opinion, not even technically correct (by correct I mean aligns with actual spoken usage). It would amount to formalizing the sentence as

For all meetings between us, if said meeting is at a past time and it's the first meeting (i.e. before all other meetings), you promised at that time to give me all your money.

Which is indeed vacuously true, if there have been no past meetings, or even if the meetings aren't well-ordered in time :). On the surface this is a perfectly good interpretation, but it doesn't really align with real usage (though I would love to see an example of "when" + past tense being used this way, e.g. in a legal document).

On the other hand, most people would interpret "when" + past to assert that the event actually happened, which in this context means

I have met you before, a "first meeting" can be identified, and at that first meeting, you promised to give me all your money.

Or even more formally

There exist meetings between us at a past time, there exists such a unique meeting which is first, and, for all meetings, if said meeting is indeed the first, you promised me at that time to give me all your money.

And this can be reduced to

There exists a unique past meeting between us such that [it's first, and you promised to give me all your money at that time].

I think this interpretation is most closely aligned with how "when" is actually used in practice. "If" feels different, though. It can act as simple logical implication, logical equivalence, or anything in between, so it may be more interesting to study. Also note that all of this doesn't apply to "when" + simple present, which acts very similarly to "if".