It is permissable to use a san tee on it's side however
(1) The maximum permitted fixture unit value of a 2" vent is 24
(2) The combined cross section of vents through the roof need to equal the cross section of the main sewer. For instance if you have a 3" main sewer line (7.065) the cross section of all vents penetrating the roof must be equal to or greater than 7.065. A 2" vent equals 3.14, a 11/2" vent equals 1.766. Technically speaking you would need at least (1) 2" vent ( the minimum size of a toilet vent) and (3) 11/2" vents.
A 4" main sewer has a cross section of 12.56, therefore you would need the equivalent of (1) 4" or (4) 2" vent penetrating the roof.
If you are talking soley of 1 bathroom then all on (1) 2" vent will be OK as long as it and the remaining vents (other baths, kitxhen, laundry, ect) are equal to or greater than the main sewer.
Hope this didn't totally confuse you but sometimes people demo the existing plumbing then without realizing it delete an additional roof penetration that had the purpose of fulfilling the cross section requirement.