How did Alisaie know? [Before the Fall]

[Nalien]

To quote Tataru:

"Anyway, after Yugiri found me, we traveled to Vesper Bay and met up with Urianger. When we told him what happened, he used his magicks to weave a glamour around the Waking Sands."

So, yeah, I don't think that's quite right...Urianger didn't cast the glamour until after the fact.

Well, remember that Tataru and Yugiri were already in Coerthas when we arrived. It is reasonable then, to assume that the work was going on to apprehend the remaining Scions before the banquet. Yugiri and Tataru likely arrived in Vesper Bay in time to inform Urianger (and thus Alisaie) before or during the events at the banquet. The alternative is that the Scions were being apprehended at the same time Tataru was being chased, and Yugiri and Tataru traveled from Limsa, to Vesper Bay, then to Coerthas, in the moments we were making our escape... While we were using our own personal taxi service (the Enterprise)... I don't think that journey would be possible, I think they got to Vesper Bay in time to tip Urianger/Alisaie off to what was going on and that's why Alisaie was able to send help.