Assalamu'alaikum Warahmatullah
What is the ruling on singing "I vow to thee, my country" (without the instrument)?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.

There is nothing that necessitates the prohibition of repeating this phrase, and it is not a vow of worship to other than Allah.

We advise you to avoid whispers, ignore them, and stop being preoccupied by them.

Allah knows best.