Offset bench vise are handy to have in the shop and they are not cheap. Well worth to have it repaired. I hope you find a good solution or someone with experience welding to fix your vise.
The other day I dropped my small drill press vise and it broke in half. I had some ERCuSi mig wire (Blue Demon brand) and some argon gas so I decided to give it a try. It was my first time using this wire and I was looking forward to give it a try. I did a good bevel around the cracked edge. The wire is only .024" dia and I was trying to weld some 1/4" cross section, so the setting on my mig welder were all max out. It didn't flow so well on the first pass, but on the second pass it seemed to have enough heat to flow nicely across the two sections. I then proceeded to grind all the excess weld, it looked ok, no large pores and a decent fillet, but a lot of black soot around the weld. Oil burn off??? I clamped a pipe in the vise and it seemed ok, until I squeezed it a tad more. LOL it broke in half. With a cold chisel I was able to chip the weld off the cast iron clean, like mentioned its more like a glue, no fusion between the metals. If I ever get the urge to try to fix it again I would do has suggested, a good oil burn off with a torch, sand blast the crack and a pre heat before welding. And I would not use my mig with that small wire to weld a 1/4" cross section, I would instead try to braze it together. I do have a small bench vise which was torched brazed 20 years ago and it's still holding together. Maybe because it was done by a professional.