Look at these adjectives:
Note that to form the masculine plural, adjectives ending in -eau add -x, and vieux doesn't change.
As for the feminine plural, it is regular and simply takes an -s.
The masculine forms bel, nouvel and vieil (when in front of a vowel) become the regular masculine plural beaux, nouveaux and vieux.
Examples and resources
"Plurals in French" by www.frenchspanishonline.com