I have the Voluntary Price for Small Consumers (VPSC - Formerly LRT). Is it in my interest to switch?

Yes, it's in your interest to switch. With the ONE Rate you have an indefinite discount on the fixed charge. The ONE Rate price is lower, and you will also avoid possible market variations in the VPSC price. 

With Endesa's ONE rate you contract only what you need, with no obligation to contract an additional service but with the possibility to do so if you need to.