The Tortoise trust, British Chelonian group the world Chelonian group have all spent years researching the use of viv's and all have come to the conclusion only very experienced viv users should even attempt to raise a tortoise in a viv.
Viv's don't circulate the air evenly enough, even with vents and fans. Tortoises need much air circulation to prevent respiratory conditions such as RNS and Herpes which are difficult to treat, can cause perminant lung damage or even death.
They don't allow a big enough temperature gradient, to allow the tortoise to thermoregulate. Temperatures need to vary greatly hot end to cold end.
Viv's aren't designed for tortoises so don't come in big enough sizes for a tortoise to be healthy and happy.
Also heat mats can and do burn Tortoises terribly
Tables are cheap to make (under £15) for a decent one and can be as big or as small as you want and do keep the temperatures warm enough for tortoises.
This is what the tortoise trust say about the use of viv's ( the link is aimed at people looking for tortoise indoor accomodation, if you would like a more specific report, email Andy Highfield at the TT, I guarantee you he will be more than happy to give you his many years of research reguarding the use of viv's
Tortoise Trust Web - Indoor Housing & Vivarium Design