Vagues is an adjective here so has the same meaning than in English: vague, imprecise, blurry.

The fracas de leurs écumes evoke the huge sound made by waves crashing into rocks so there is definitely a poetic pun here. There are no real waves as écumes is referring to the amours. Les écumes des amours means the remains of these old romances.

The refrain of this song is telling that the singer is deaf and probably blind which is mitigating the repeating “you are beautiful when …” statements.

It reminds the well known quote l’amour rend aveugle (love makes you blind).

It is far from useless to consider the whole piece in order to gain a better understanding, so here is its copy below.

        T’es belle quand tu souris
        T’es belle tout le temps
        T’es belle même quand tu ris
        Et qu’on voit tes dents.

        T’es belle quand tu rougis
        T’es belle en toutes circonstances
        T’es belle même quand tu fais une blague pourrie
        En plein dans un silence.

        T’es belle quand tu te trompes
        T’es belle quand tu t’entêtes
        T’es belle, je m’en rends compte,
        Même quand t’as l’air bête.

        T’es belle quand tu t’énerves
        T’es belle sans te mentir
        T’es belle même si, quand tu t’énerves,
        C’est pas le moment de te le dire.

        Déjà du temps de mes amours vagues
        J’étais sourd au fracas de leurs écumes
        Et comme l’amour m’a toujours fait des blagues
        J’dois être aveugle, je présume

        T’es belle quand tu bois
        T’es belle quand t’es bourrée
        T’es belle quand tu marches pas droit
        Et qu’tu sais plus parler.

        T’es belle quand tu dors
        T’es belle quand tu ronfles
        T’es belle, c’est ça qu’est fort,
        Même quand tu m’gonfles.

        T’es belle mal réveillée
        T’es belle et je te jure
        Que t’es belle même quand t’as été coiffée
        Par tes couvertures.

        T’es belle quand tu m’reproches
        De pas être honnête avec toi
        Mais t’es belle même quand tu te trouves moche
        Alors je le garde pour moi

        Déjà du temps de mes amours vagues
        J’étais sourd au fracas de leurs écumes
        Et comme l’amour m’a toujours fait des blagues
        J’dois être aveugle, je présume

The stanza under question is a leitmotiv in the poem and it’s essentially disconnected from the concern of the rest of the poem except that the reflection on his past love life that is formulated in it occurs in the unrestrained declaration of his admiration for her beauty, that being tantamount to a declaration of his love; this declaration is the concern of the whole lyrics bar stanzas 5 and 10.

The nature of the word “vague” appears to be that of the adjective “vague”, meaning as in English “not well defined, not well asserted” (“amours vagues”, love somewhat like puppy love we might think) and that has nothing to do with the noun “vague” meaning “wave” in English. “Écume” is “foam” in English, however, here, in the poet’s mouth, it’s not foam associated with breaking waves but it is figurative foam associated to his love life, specifically “his vague loves”. Just as there is a characteristic noise accompanying the apparition of real foam on top of breaking waves, a figurative noise is made by this figurative foam and that is “fracas”. It seems then that he is identifying the imperfections of waves with what real loves implies that is less than ideal.

Since he is referring to his past love as “amours vagues” we can understand that this is not the case now and his love for her is not an “amour vague”. Nevertheless, there is still associated with it the “noise of its foam”, but he tends to believe that he is still totally impervious to this figurative parasitic sound, this nagging hissing. While he is aware of an unabated love that is motivated by beauty he is also making out this feeling against the background of this hiss and he tries to reconcile his mixed feelings in resorting to the popular notion that love is blind and that this ambiguous stance of his that makes him uneasy is really what this piece of wisdom expounds.

What a great few lines!
I especially like the many layers of references and sensory experiences.
For instance, the idea to be blind by love is a rather common one, but in this text it takes a more profound meaning through the distance put between “amours” line 1 and “aveugle” in line 4.
“Ecume” also for me refers to Boris Vian “L’écume des Jours”, a surrealist love story.
Finally, if you ever, on a windy day, stand on a pebble beach like the ones in Normandie or South England you will clearly get to hear the noisy fracas of the waves crashing on the shore with heaps of foam.

As a reference, pls see https://www.cnrtl.fr/definition/academie8/fracas