Assuming Gianni’s isn’t available for this trade.
Celtics give up the best player in the trade but they get a package the makes sense for the short and long term. They take on an expiring stopgap pf in Barnes. They also get cam Johnson who should be an ideal fit next to Tatum as an off ball shooter on the wing. The #2 pick is the asset the spurs will need to give up in order to get the two way all star at a position of need for the spurs. The Celtics give up 28 to the nets who are ultimately giving up a better player now then they are getting back.
The spurs title window starts immediately and they still have most of their picks to add to the team. They will trade down from 14-19 as part of the trade which shouldn’t be that big of a deal if it means locking in title contention for the next 4-5 years.
The nets trade into the lottery and pickup an extra first to further move up a couple spots if they find a willing trade partner. Vassell is still only 24 and could end up outplaying his contract. It’s a gamble but this is the high upside swing they should go for when there isn’t a clear difference maker on the roster yet.