I want to buy some of the vendor weapons from TTK, so if I uninstall RoI I will be able to obtain them again. However, won't my light level go down to 335 (max light in TTK)? And if so, what happens when you go back to RoI? Edit: I am going to make a new PSN to access old weapons.
Unfortunately there is no way to revert your game to a previous version. In order to even play Destiny you must have the most up to date version. The DLCs you install are just license keys which unlock content for you. Uninstalling a DLC will not affect what the vendors have for sale.
Vendor weapons are control on Bungie's end. Uninstalling ROI will not allow you to get the weapons/gear vendors had during TTK. Every so often, usually with larger updates, vendor supplies change.