Vidic is one of a number of experienced United players who are out of contract this summer, and with the Red Devils showing an interest in both Luke Shaw and previously Leighton Baines, the future of Patrice Evra is in doubt.
Evra has been linked with a move to former club Monaco, and Moyes was unable to confirm whether or not Evra would remain at Old Trafford.
"It [losing three senior defenders] could have happened at any time," Moyes is quoted as saying in the Daily Mirror.
"They're great pros, working hard and still here. With two of them it's still hypothetical. Vida has made his decision, he wants to be straight and stop speculation.
"I speak with Patrice a lot and I know the people he works with. There are things in place and we'll tell you when we want to.
"He's a great role model, a great figure in the dressing-room and he's been a great player for United. His type is the type we want to bring to the club and have at the club.
"Time is the one thing [you need]. You hope in time we'll have a side that competes."
It remains to be seen whether United will look to bring in defensive replacements or place their trust in the younger members of the squad.
Regardless of whether or not Evra maintains his stay at Old Trafford, it is thought that United will again be in for Southampton full-back Luke Shaw, as they look to beat Chelsea to the English prospect's signature.