To make an impact, the Vegas train would have to get closer to LA than just Victorville.
Being able to use CAHSR infrastructure is thus an integral part of the plan.
In other words, CAHSR has to be built first.
I would think that Brightline will ultimately build a new Los Angeles area station with TOD that is closer to the center of the region. Victorville and Palmdale will become intermediate stops if they choose to use the same route Xpresswest was planning on building to Palmdale. Victorville to Las Vegas would be similar to building Orlando to WPB. To make it work, Brightline has to build further into the LA basin.
As far as CA HSR having to be built first, if Brightline goes with 125mph equipment they can negotiate agreements with the freight railroads to use their routes to gain access into the LA basin. The grapevine route is more likely due to the nature of Cajon Pass. The PR announcing the purchase of Xpresswest specifically said no decision on trainsets or final track design had been made yet as it was all under review. Even at 125mph maximum authorized speed, running time of less than 2 hours can be possible. Since CA HSR will not be operational into LA for at least 20+ years, I doubt that Brightline is too concerned about interoperability issues with CA HSR at this time. As long as they design it to allow for future electrification then they can use 125mph maximum speed diesel trainsets for the first 20-25 years. Once it is electrified, those diesel trainsets can be transferred to other Brightline routes in place by that time.
Edit: Also, as pointed out by others, Brightline is looking to cut the cost in half (from $7B to $3 - $4B). If they don't require electric 170mph trains they won't use them. Unlike CA HSR, Brightline has to make a profit and can use trainsets that aren't the fastest and latest technology available.