Rogue Sneak Attack ability or when do I have the advantage and with which weapon

Once per turn you can deal an extra 1d6 damage to one creature you hit with an attack if you have Advantage on the roll and the attack uses a Finesse or Ranged weapon. The extra damage’s type is the same as the weapon’s type.

I have a hand crossbow and a dagger.

I have no advantage to start with and I am by myself.

I shoot the hand crossbow.  It hits, I have vex mastery so I now have the advantage.  Does my dagger hit roll now have advantage or is it next turn?

Assume I get advantage with a dagger the same turn,  Do I have advantage next round shooting the crossbow (ignore loading  issues)?

If you score a hit with a Vex weapon, your next attack against that creature has advantage as long as it occurs before the end of your next turn.

If you are making 1 attack per turn, and if a Vex weapon hit first, then your attack on the next round would have Advantage. If you fail to take an attack on that next round, you do not continue to gain advantage on further rounds without re-applying a hit from a Vex weapon.

If you are making 2 attacks per turn, and if a Vex weapon hit first, then yes the second attack of that turn gets Advantage.

If you are making 3 attacks per turn, and if a Vex weapon hit first, then you would have advantage on your 2nd attack, but not your third. 

Think of that 'attack with Advantage' as being a coin made of ice. You earn the coin when you hit with a Vex weapon. You can then spend that coin to take an attack with Advantage, but must do so before that coin of ice melts. It melts away at the end of your next turn.

”If you hit a creature with this weapon and deal damage to the creature, you have Advantage on your next attack roll against that creature before the end of your next turn.“ - Vex

It only works for the next attack roll, so if you attack with the dagger while it has the Vex's advantage, the crossbow wouldn’t have advantage next turn.

Quote from Character77006 >>

Once per turn you can deal an extra 1d6 damage to one creature you hit with an attack if you have Advantage on the roll and the attack uses a Finesse or Ranged weapon. The extra damage’s type is the same as the weapon’s type.

I have a hand crossbow and a dagger.

I have no advantage to start with and I am by myself.

I shoot the hand crossbow.  It hits, I have vex mastery so I now have the advantage.  Does my dagger hit roll now have advantage or is it next turn? 

The Vex property says that you have advantage on your next attack roll against the same target before the end of your next turn.

So, after you hit with the crossbow, the next attack you make against that same target, regardless of which weapon you use, has advantage. That could be an attack with your dagger, or another attack with your crossbow, or an Unarmed Strike, or a spell attack, or whatever other kind of attack you might make. It's the next attack you make before the end of your next turn. That might be on this turn or next turn. 

Quote from Character77006 >>

 

Assume I get advantage with a dagger the same turn,  Do I have advantage next round shooting the crossbow (ignore loading  issues)? 

No, because it's only the next attack you make. Once you attack with the dagger with advantage, the advantage from Vex is used up.

Thank you.