Vermin Lord: Verminkin attack roll calculation

From Grim Hollow;

"You can comprehend and verbally communicate with Tiny Beasts.

As a Magic action, you can expend a spell slot to summon vermin swarms. You summon a number of swarms equal to the slot’s level for 1 hour. Each swarm is summoned to an unoccupied space you can see within 30 feet of yourself. A swarm uses the Swarm of Vermin stat block.

In combat, each swarm acts during your turn. It can move and use its Reaction on its own, but the only action it takes is the Dodge action unless you take a Bonus Action to command it to take an action in its stat block or some other action.

Once you use this feature, you can’t use it again until you finish a Short or Long Rest."

What I want clarification on is the attack roll of these critters.;

"Bites. Melee Attack Roll: Bonus equals your spell attack modifier, reach 0 ft. Hit: 2d6 + 1 + the spell’s level Piercing damage."

If the spell casting Modifier was, say, "+3" would the calculation for the attack roll be 1d20+3, or would it be given the treatment of a melee spell attack, which applies your proficiency bonus as well?