"My light hood has a sticker that says 30" / 20watt. Does this mean it is
only safe to use a 20 watt bulb in it and that I will need to buy a new
hood, specially made to handle VHO bulbs if that's what I want to go with?"

If the VHO is marked other than 20 watts, it won't work with the
ballast in your hood. You are most unlikely to find a hood with a
ballast designed for VHO. (In fact I've never seen one with even
a rapid-start ballast here in Canada!) If you know your
electricity you could try swapping the ballast for one designed
for the tube you want, but it may be too large to fit and it will
probably overheat if the wattage is higher than 20.

I assume you want to increase your light level, but don't know
how experienced you are as a handywoman. Can you modify your cover
to take a complete reflector fixture designed for the higher
lighting you want?