Since this is a minion insubstantial cant serve its usual funtion of reducing damage, therefore I would eitlher treat it as some value of damage reduction or increase the defenses to make it tougher to hit. I would lean toward increasing the defenses by about 5 across the board.
I would very strongly advise against that. +5 to all defences is a massive increase, especially for a level 1 creature.
Or you could model the insubstantial by giving the minion a save when it takes damage. Pass no damage, fail take damage and the minion dies as normal.
I tried this approach once. I had minions that survived longer than standard creatures because they kept making saves. I havn't repeated the ability because I didn't think it worked that well.
With this approach you could then model the vulnerability with either a penalty to the save, or treating the save as an automatic failure, i.e. an attack to which it is vulnerable that hits kills it outright just like a normal minion dies when hit.
That makes sense and might be worth considering.
What I find myself thinking with the initial question is why bother?
Question:
What is the difference between a normal minion and an insubstantial minion?
Answer:
A normal minion goes thud when it hits the floor.
Seriously though, I would maybe give them a reaction power that did something flavourful that represented their insubstantial quality and leave it at that.
Minions die fast, and no-one weeps at their graveside when they are gone. As I have often said, it is a hard life being a minion.
If you think an encounter with insubstantial minions should be harder then add a couple more of them in at no increase in xp value. Also set up the encounter so that they can make maximum use of their insubstantial nature. Give them phasing as well and have them charge through walls, then try and escape to do the same thing again. Usually if a minion manages to do damage once before it dies it has lead a full and productive life, if a minion manages to deal damage twice then it is a hero.