many times people feel like they shouldn't drop their teammate if they have 2,900 creds left because they don't want to ruin their own economy, but logically speaking, having your teammate with vandal/phantom is better than pistol because it ruins the team's economy. you should be buying with your team and saving with your team, regardless of how good/bad they're doing, otherwise building up momentum for the win will be a lot more difficult.
just make sure you buy your team no matter what :)