I have repaired several tail cracks in big and small vises like this Reed 109 ( 9″ jaws), a big vise. There is several ways to repair these cracks and this is one way I feel is a good fix.
First I drill a 3/16 hole at the end of the crack, this stops the crack from spreading. I then use a carbide ball burr and groove out the crack. I like to groove at least a 1/4″ deep and at least a 1/4 wide more does not hurt.
The next step is to clean the crack as best as you can, The vise should have been de-greased before starting the repair. Heat is a good way to clean the crack. I like to heat the area to at least 400 degrees and use a SS brush to remove the burnt crud.
After I spent about a good 1/2 hour of heat I then let it cool. Next step is pinching the tail down to bury a Socket Head Cap Screw. I use my milling machine and my toe clamps to pinch it to the correct thickness. I have gauge blocks that I can stack up to the same thickness as the front of the square slider. Using a dial caliper works too. Your drill press has tee slots to clamp down the slider too. You can do this repair without a milling machine.
I slow down at this point and take careful measurements to be sure the cap screw is centered in the thickest area of the web and as far back as you can. I like to drill the screw body to the center of the square slider and past the crack, then drill the tap drill size at least 3/4″ more. I have long series taps and if working on a bigger vise you will need one to.
I do not use oil on the tap, you do not want to add any contaminates before welding or brazing. Use a magnet to remove the chips, air is not enough. Use a end mill or a counterbore cutter that matches the socket screw. I go deeper by a 1/4″ to add a plug.
Add the screw and tighten it as tight as you dare. Check to be sure you did not bottom out. Add a plug and peen it in tight and be sure it is higher so you can sand or cut it down to match the side of the slider. Two screws are better then on if you have the room. Ready for welding or brazing.
I like to weld since I have several different rod choices to use even though I feel brazing is as good or even better. First you have to reheat the casting, I have a pyrometer I purchased off of e-bay for around $30. Heat slowly up to 400 to 500 degrees.
All sorts of welding rods can be used, I have Inconel, Nickle, Silicon Bronze and Eutectic 224 Tig rod. Brass and Silver Solder works too. I have found through experience that sometimes the 224 rod works or the Inconel bonds with the casting. Just never know until you try it. I keep the heating of the casting going till I am ready to weld. I like to tack weld in several spots before pouring on the heat and welding.
After welding and reheating to over 400 degrees I like to peen the weld, this does two things, fist it stress re-leaves the weldment and second it swells the material around the weld so when you remove the weld you do not see sink at the eutectic zone of the weld. Heat again to over 400 degrees and cover with a blanket or what I use is old leather gloves and cover as much as I can to slow the cooling. Clean off the weld with what works for you, I use my mill and your done.