Roughly bruise the mint and rosemary so that they will release their oils.
Put the herbs in a non reactive pan and pour over the vinegar.
Bring up to a simmer but don't boil, simmer for 20 minutes then remove from heat.
Let the pan cool and leave it overnight covered with a tea towel.
Strain out the herbs and pour vinegar into sterilised bottle.
You can add a fresh sprig of rosemary to decorate but you must always make sure that this is covered in vinegar or it will spoil.