Pretty simple problem, probably a pain to fix.
The AI does not care about the artefact planet in another star system. At all. The lowdown: I got to the victory condition planet at about 2 hours into the game. (Mind you this is on the largest map setting possible without building your own map with 10 ai players set to hard difficulty.) When I got there the defending fleet hadn't been touched, so I cleaned out the defenses and took over the planet from the neutral faction. A half hour later, not single vessel came to that star system. (I had a scout in the star's grav well). No scouts, no massive assault fleets, nothing. Granted, I did have the planet heavily fortified, but none of the ai players would know that.
I can probably think of two issues why the AI didnt attack me.
1: They didn't know where the artefact was, and/or didn't have long range jumps researched.
2: They didn't prioritise the planet as a victory condition. In fact, despite knowing I was going to win the game (and with 90% of my fleet destroyed by the neutrals), I wasn't even attacked. Instead my neighbors went to culture warfare. (Although, I have to praise this move, my main rival lost two large fleets to two heavily defended worlds, and switched culture, effectively removing several of my planets from play.)
Even if the above was true, when a victory condition has begun its countdown, either with research, occupation, or any others with a timer, the AI should try it's very hardest to stop me. After all, they're playing to win.... aren't they? If they didn't know where the planet was, the game needs to reveal the star system and the planet, and pretty much give the AI a push or shove to try and take it asap. At least scout it, anyway! Perhaps give AI (and humans, maybe?) full vision of the star and it's artefact planet?
On normal I can see not caring so much, but on Hard and above it kinda needs to be... you know... hard. I even set the win speed to the slowest setting because I figured they wouldn't attack it in force. But to completely ignore it?
I imagine this isn't an issue on single star system maps, or atleast, not as great a one. Can anyone confirm that they target the artefact planet directly with all it's might, instead of just raiding it like they would any other normal planet?
Finally, as an aside, when I won as the TEC loyalists, I heard a vasari voice over during the victory screen. Small thing.
TL,DR; AI doesn't attack or even go after artefact planets in multiple star-system maps.