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I agree with Lucy's comprehensive response. Also to say, as a direct answer to the original question, that making any sort of hole in sand, especially water bearing, probably fairly uniform sand, will require casing. Jetting could be effective and quick - you'd need an air compressor, ideally - but I guess that, as you have the rig, you want to use it.
Having looked up the Vonder rig, I see that it is a hand-operated tripod device. I assume that you are using some sort of baler tool with a flap valve or some way of retaining loose sand (as described by Lucy). You should be able to work the tools inside a casing, which, if conditions are right, might penetrate under its own weight as the drilling proceeds and the sand is excavated from beneath the casing. more likely, you'd have to add force to drive the casing down, as described by Lucy. Therefore, steel casing would probably be better as you could hammer it if necessary - but if you do have to hit the casing, use a collar to avoid damage to it - you may have to add casing sections.
Don't know how much this helps.