By Dan Weil Inventory and vacancy rates have plunged in Manhattan. Rents are seen headed higher nationally. The list of inexpensive places is ‘shrinking.’ If you’re a renter, you probably know first-hand what’s happening to rental rates. They’ve been going up over the past year. That’s a big deal because as you also know, rent likely represents your biggest regular expense. Rents for primary residences soared 0.6% in February over January, the biggest gain since 1987, the government reported on Thursday. Rental data are reported with a lag, so that number will almost certainly rise going forwa…