Unless your line manager authorised the secondment (in which case, why PIP?), it wasn't on official assignment, it was just a personal request from someone who happened to be in a higher management position.
Telling the VP to get stuffed, you're only going to do what comes through the proper chain of command, is just going to get you fired immediately rather than PIPed.
Not saying there wasn't a way through this that could have led to a positive outcome, but the key takeaway isn't "do the dirty work", it's "make sure what you're doing has an impact, and has visibility from the people in charge of personnel decisions affecting you". Less work but greater visibility > more work and worse visibility. Part of your job is ensuring visibility or you will get shafted.
Unless your line manager authorised the secondment (in which case, why PIP?), it wasn't on official assignment, it was just a personal request from someone who happened to be in a higher management position.