From the Vistana website pertaining to Lagunamar: Rollaway beds, cribs and other special request items are limited in supply and are fulfilled on a first-come, first-served basis. Charges apply for rollaway beds and some other special request items. Rollaway beds are limited to one per villa and are not available in the studio premium villa due to space limitations.
From the Marriott website pertaining to Lagunamar for the Studio and 1-bedroom Villa:
Since they conflict I recommend you call the Lagunamar concierge and ask the question. They have a toll-free number: 844-260-9083