Treasures you control will keep any abilities they previously had while they’re creatures.

A card, spell, or permanent is an outlaw if it has the Assassin, Mercenary, Pirate, Rogue, or Warlock creature type. It doesn’t matter if it has more than one of those creature types; as long as it has at least one, it’s an outlaw.

If an ability refers to an outlaw or whether a player controls an outlaw, it’s referring only to permanents with one or more of the creature types specified above. Notably, it’s not referring to any spell or card not on the battlefield. However, other abilities may refer to an “outlaw spell” or “outlaw card” in a zone other than the battlefield. Those abilities refer to spells and cards with one or more of the specified creature types.

Outlaw is not a creature type. If an effect asks you to choose a creature type, you can’t choose outlaw.

If a Treasure you control is already a creature when Vihaan’s second ability resolves and you choose to apply its effect to all Treasures you control, that effect will overwrite any previous effects that set that creature’s power and toughness to specific numbers. Effects that otherwise modify its power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.