Does a pilgrim's wearing of an ihram other than his own for performing Umrah on behalf of a deceased person, due to vomit on his original clothes after passing the Miqat, invalidate the Umrah?

Performing Umrah on behalf of a deceased grandfather is permissible, provided you have first performed Umrah for yourself. If you have not done so, then the Umrah will be counted for you, not for your grandfather. There is no harm in changing soiled Ihram garments for clean ones, and this does not affect the validity of the Umrah, provided you do not wear regular clothes in between. If you wear your regular (stitched) clothes, you have committed a prohibited act, for which you are required to offer a fidyah (sacrificing a sheep, or feeding six needy persons, or fasting for three days), unless you were ignorant of the prohibition, in which case there is no penalty upon you.