The Vector should drive 4-ohm speakers with no problem, they were OK units in their day. One thing I would do, and I do this with all vintage solid-state amplifiers I drag home from thrifts/yard sales/the curb/whatever is to set my DMM (digital multimeter) to DC Volts, turn the volume off, make sure the speaker switch is on, and measure across the + and - speaker terminals for each channel to check for DC offset. It should be no more than 500mV at absolute worst, and ideally should be under 200mV (closer to 0 the better.) If there is little to no DC at the speaker terminals, you're safe to proceed to the step of listening to the unit and testing for scratchy pots/switches/etc.
Vector Research was an almost-sister-company of NAD back in the day, IIRC...
Todd in Cheesecurdistan