Can I Palp one evade and then reroll the rest using the Vanguard Title (as the die changed by Palpatine cannot be modified)?

Yup that would work fine.

Any precedence on this one?
The title says you may discard to reroll all evade dice, which reminds me of Old Han's pilot ability..

Yup, that's the precedent. Not that precedent is necessary, you reroll all the dice you can reroll. You can't reroll the Palped die.

I don't think it would work. You are not allowed to reroll only those dice that were rerolled already - hence Han was OK to reroll everything but results rerolled, i.e. by Predator. Though, if you spent your focus token to change some results from eye to hit, you still have to reroll them in case of Han Solo or FOV.

On ‎01‎/‎12‎/‎2017 at 8:57 PM, thespaceinvader said:

Yup, that's the precedent. Not that precedent is necessary, you reroll all the dice you can reroll. You can't reroll the Palped die.

Compare Han with Predator, which was a common build. By rerolling a die with predator you 'locked' that die into the result it was now in, such that a subsequent reroll ability couldn't touch it.

Palpatine's "may not be modified again" clause does much the same.