I know that sanatana dharma is the oldest religion. But are there any evidences or any mentions of other religions in the vedas? If yes,what were the religions?
Rig veda I.110 says about Ṛbhus
When, seeking your enjoyment onward from afar, ye, certain of my kinsmen, wandered on your way, Sons of Sudhanvan, after your long journeying, ye came unto the home of liberal Savitar.
Savitar therefore gave you immortality, because ye came proclaiming him whom naught can hide; And this the drinking-chalice of the Asura, which till that time was one, ye made to be fourfold.
These mantras are talking about existence of different methods/religious rituals in Vedic Era, other than Spirituality.
Here immortality does not indicate a boon to live forever,which in the subsequent literature described as chiranjIvatva, but only permanent BLISS emanated out of SELF REALISATION.
In Ramayana, Sri Hanuman was described as the follower of dakshinAchAra - Vedic way of life.
अञ्जलिं प्राङ्मुखः कृत्वा पवनायात्मयोओनयो | ततो हि ववृधे गन्तुं दक्षिणो दक्षिणां दिश्म् || ५-१-९
Hanuma, follower of dakshinAchAra, turned towards east, saluted his father, the Lord of Wind and increased his body to go towards southern direction.
In this sloka दक्षिणो indicates follower of dakshinAchAra, दक्षिणां दिश्म् indicates in Southern direction.
So there existed vAmAchAra - the way of life that opposed to vedic way of life.
While giving away her ear rings, the queen of king Paushya, she warns Utanka about King of Serpants, Takshaka, who is waiting to steal those ear rings.
'These ear-rings are very much sought after by Takshaka, the King of the serpents. Therefore shouldst thou carry them with the greatest care.'
On the road Utanka perceived coming towards him a naked idle beggar sometimes coming in view and sometimes disappearing. And Utanka put the ear-rings on the ground and went for water. In the meantime the beggar came quickly to the spot and taking up the ear-rings ran away.
Narrating appearance of Takshaka in the form of a naked beggar, might be a pot shot against Jainism, which existed at the time of composition of Mahabharata in Classical Sanskrit.