Inner tube valve cut from stretched rim tape?
Several fixes


If you have tools, smooth off the cut edge of the hole where the butyl rubber tube touchs it.  A tiny file, or careful use of sandpaper to take the sharp edge off would help.
Reinforce the rim tape with paper masking tape.  Two layers around the whole rim would reinforce the existing tape, with overlap 100mm either side of the valve hole.  As long as the tape sits in the well/valley and has no way to interfere with the tyre's bead/hooks then its okay.   To get the valve through, cut three slots across the hole making an asterisk pattern, and leave the flaps there to fold into the hole around the valve stem
Note, patching a tube hear the valve stem is challenging.  Consider buffing the area of the hole really well, and allow the vulcanising fluid to set for 10 minutes before applying the patch, with pressure.   If the tube doesn't test okay, then just replace it.   Tubes are consumables too.


The best fix is new rim tape, which is not expensive, and the newer sorts don't stretch much around the valve hole.  You might still need a new tube.