Yes. we need to see the VU board schematic or at least its PSU section to detemine whether it can run from a higher voltage or if it really needs its own isolated 9VAC supply.
If it does need its own isolated AC supply, assuming the toroidial transformer has an open center (i.e not potted). try the overwind experiment Brumby suggested, only to make the maths easy, wind TEN turns of hookup wire through the center of the toroid (not six as suggested). Measure the voltage from that and you can calculate how many turns are actually required for 9V AC.
Anything under about 100 turns is no particular problem to wind. If its going to need more turns (unlikely) I'd reconsider.
Mark the two ends of the temporary winding were they cross the same edge of the toroid, unwind it and measure the length between the marks. Calculate the length required for your 9V winding and spool off enough (with about 10% extra + 2x leadout wire length) thin (but not too fine) magnet wire onto a spool that will fit through the core e.g an empty cotton reel.
Thread thin sleeving (e.g. stripped from 1mm solid wire) long enough for the lead length you want over the end of the magnet wire with 1" of wire emerging which should be folded back and taped, and cow hitch a 6" length of heavy unwaxed linen thread round the other end of the sleeving. Cow hitch another linen thread round the magnet wire just past the end of the sleeving. Temporally tape the sleeving to the outer side of the toroid, with about a 1/4" gap between the tape and the nearest linen thread. The cow hitch should be centered on the face. Lay both ends of both linen threads along the toroid face in the direction of the proposed winding and secure the far end with tape.
Wind twenty turns with the magnet wire (or half the required turn count, whichever comes first) tight and close packed but not overlapping, then untape the linen threads, pull them as tight as possible, fold them back towards the start of the winding and tape them out of the way. Continue winding until there's 20 turns remaining (or if less, already don't continue). lay two 8" loops of linen thread along the outer side of the toroid, looped end away from the winding, one on the center line and the other a bit towards one edge, in the direction the winding will be coming from. Tape the tails down on top of the winding and tape the loops down as far round the toroid as they reach stretched tight.
Carry on winding, over the top of the new linen threads till the desired turn count is reached. Temporarily tape the winding end, and power the transformer to check the new secondary voltage. 5% high is good as it will drop a bit when loaded. Adjust the turn count if its too far off.
Cut the wire end a few inches longer than the first one. Thread a matching piece of sleeving over it, untape the loop ends of the linen thread, form cow hitches with them round a drinking straw, and thread the wire end with sleeving through the drinking straw and pull out the straw so the cow hitches are round the wire and sleeving end just like the starting end. Pull the free ends of the linen threads to tighten the cow hitches and lightly tension the wire inside the sleeving and fold back the end.
Make sure everything is snugly in place and all the linen threads have been tightened then its just a matter of locking the threads in place to permanently anchor the winding and sleeving by applying enough liquid superglue to saturate the linen threads, trim off the tails and let it all set. Remove as much as possible of any remaining temporary tape. If you've got any electrical grade lacquer, conformal coating or varnish, now's the time to give the whole new winding a coat, and let it fully dry.
Reinforce where each sleeving end is attached with a short strip of heat resistant insulating tape (preferably cloth, mylar or kapton and definitely not PVC) running round the outer diameter of the toroid, then wind insulating tape over the winding, starting with a full turn before the winding and finishing with a full turn after it. The sleeving with the leadout wires should emerge between turns of the tape.
Result: a professional quality overwind 9V AC secondary.