I believe that Contador and Vino are friends, and that he'll keep his word about helping Alberto win the Tour. He also wants Contador to stay with Astana after this year, so he wouldn't do anything to mess that up.
Even if you don't believe his word, fitness level and peaks come into play. Vino has said he's at his peak fitness for the year for the Giro. If you've watched the race, that's easy to believe. If you're skeptical, he won Giro del Trentino, caught a flight after the tough Friday mountain stage, and nabbed a surprise win at Liege Bastogne Liege against people who targeted that race. He's spent a lot of time this year training at altitude on the climbs of Tenerife. He's going to give the Giro everything he has, especially on the tough mountain stages the last week.
When this race is over, he's got 33 days to rest, recover, and get back on the bike a bit so he can endure another three week event. He won't be doing massive training days on the climbs of the Tour, and he won't be as fit or race ready as he was at the start of the Giro. The whole Sky team is trying to train through the Giro to prepare for the Tour. They know Wiggins can't be at his best for both. Neither can Vino, and his highlighted part of the season ends in a couple of weeks. Riding the Tour for Contador is overtime.
Astana works together this year. They do their parts, and celebrate each other's victories.