A simple way of raising the acidity is to put the vinegar into a plastic bottle and freeze it. Then open the bottle, put it heads down into another container and let it melt at room temperature. The water will freeze earlier and needs longer to liquify again. You will see that a typical ice crystal core stays longest in the bottle, and all vinegar flows down into the second container. When all or most of the colour is gone from the plastic bottle, take it away. The liquid in the lower container is more acidic than what you've started with.
And as stated above, in addition or alternatively you can heat it mildly. The water will evaporate before the vinegar.
A more elaborate way which will also lead to more acidic vinegar is distillation. The water evaporates and goes over first, so you'll change the receiving vessel after 1/4th of the liquid has been distilled. The second and third receiver will each be more acidic, and the final rest in your still will be the most acidic. You can reach pH of around 3-4 in the third receiver. If that's not enough, distill the result again.
Please take care: after any of the above steps you'll have more or less vinegar essence. Do not apply to the skin directly, and definitely do not take internally. Also, undiluted it might bleach all kinds of surfaces, clothes etc.
