When purchasing merchandise from a vendor, what account should freight be charged to?

That should be charge to Freight In.

Technically, freight is part of the cost of the merchandise, and should somewhere, somehow end up added to the merchandise. If this is for school, you need to check how your book does it. If this is real life, it can be easier just to add it to the inventory account as it comes in, and kept track of as part of the cost. That way when it's sold and you need to know cost, it's already in there.

This is assuming perpetual method, since that's what most companies use anymore. If it's periodic method, then yes you put it in freight, which will still need added to the purchases at period end.