Think I might have answered this in another thread for you - but in case others are looking...
Hope this helps, using the .NET SDK:
//Load VDC you would like to place vapp in
Vdc vdc = Vdc.GetVdcByReference(Client,vdcReferenceUrl);
//Load template you would like to use
VappTemplate template = VappTemplate.GetVappTemplateByReference(Client,templateReferenceUrl);
InstantiationParamsType instantiationParamsType = new InstantiationParamsType();
InstantiateVAppTemplateParamsType stuff = new InstantiateVAppTemplateParamsType
{
AllEULAsAccepted = true,
name = "the name for the vapp",
Source = template.Reference,
deploy = true,
Description = "Some description",
InstantiationParams = instantiationParamsType,
};
//Method to create it created... the app returned will not be usable for a while, but you can get the tasks for it and reload to see the status
Vapp app = vdc.InstantiateVappTemplate(stuff);