Delta is in the process of buying 49% of the Virgin Atlantic from Singapore Airlines and this deal is expected to close later this year.
When this announcement was done back in December, it was expected that the airline would also shortly join the SkyTeam alliance. You can read more about this on Bloomberg article that you can access here.
Now, this has been put on hold. Virgin Atlantic may join the alliance, but it is not priority at this time. The airline didn’t join Star Alliance when it was 49% owned by Singapore Airlines.
The Virgin Atlantic would have been and still likely is at some point a good choice to become a SkyTeam member airline.