But is there any verse in the Vedas that explicitly says the Devas protect the Vedic civilization from Mleccha aggressors?
Yes, it is the duty of Lord Indra to protect the Vedic civilization from attacks and destruction by the armies of the Mlecchas, Asuras, and Rakshasas.
From the Rig Veda Samhita:
3.34.9 He [Indra] gained possession of the Sun and Horses, Indra obtained the Cow who feedeth many.
Treasure of gold he won; he smote the Dasyus [mlecchas], and gave protection to the Āryan colour.
Here is another verse that talks about Indra launching his lightning bolt and destroying the military forts of the Mlecchas:
1.103.3 Armed with his bolt and trusting in his prowess he wandered shattering the forts of Dāsas [mlecchas].
Cast thy dart, knowing, Thunderer, at the Dasyu; increase the Ārya's might and glory, Indra.
1.104.2 These men have come to Indra for assistance: shall he not quickly come upon these pathways? May the Gods quell the fury of the Dāsa, and may they lead our folk to happy fortune.
Lord Indra is the personal recipient of the Yajnas performed by the Vedic Aryans, so he has a vested interest in making sure the Vedic civilization is guarded from destruction.