See if you can find a composition (material) gasket, or use a couple of hole punches to
make your own composition gasket from some 0.032" gasket material.
The reason the "o" ring tears on your iSonic is probably as Windman suggests.
The surfaces the O'ring gets crushed between are not smooth or parallel.
Try using some soap on the threads and wet the soap and put a soap slurry on the
O' ring before installing it.
Also, the vent plug does not need to be real tight.
Just make contact with the O'ring and perhaps a little more.
The way the O'ring is used on vent plugs throughout the entire WS industry is
pretty much all wrong from a mechanical and seal engineering perspective.
O'rings are only supposed to be compressed approx. 15-20% of their overall
thickness, and the gland around them is supposed to be designed so the O'ring
can "move" in the direction required to effectively seal against both inside and
outside pressure or vacuum.
Crushing the O'ring traps it and completely defeats the normal design parameters.
A flat composition gasket (like on the oil pan plug in your car) will do a much better
job of sealing in the current design vent plugs.
Hope this helps,