Is your tank a bowfront and approx. 23 inches tall? That's what I have and I'm using two fixtures. One is a compact fluorescent at 130 watts (2 65 watt bulbs) and the other is a T5 unit with 56 watts. The 56 watt unit is for the front of the tank. Having two fixtures helps to light the "bow" part of the tank. Also the bulbs are 6700K except one of them is a colormax full spectrum.
I have plants with high lighting needs and they are doing well.
It's also nice to have two fixtures because you can graduate the lighting in the tank in the AM and PM so it's not such a shock to the fish every day.
But now that I read your question again, that's really not what you are asking! I don't think you can put a VHO bulb in a regular fixture like that. But you could probably rip out the guts and fit it with VHO hardware. I really don't know though. I'm not handy like that. Plus, once you buy the upgraded hardware and tubes -- it might not cost much more to just buy a new light fixture. You can buy a 48" Coralife freshwater, 130 watt fixture for approx. $100.
Sorry for rambling. Hope you got something helpful out of it...