Sonny was described in the novel as being short-sighted. In fact, he even described himself as "short range" in the novel. Not having enough soldiers around Vito is certainly confirmation of this.

However, I would add that 10 minutes is not a long time. Is it long enough to kill Vito? Of course. Remember that this was was set back in the 1940s. There were no cell phones and instant communication like today. It could have been that there were not many readily accessible pay phones nearby. And if one of the soldiers did manage to slip away, they may have driven back to the Corleone compound instead of risk staying on the hospital premises.