There is a story where vamana promises to aditi that he will always take birth from her womb, as kosalya was incarnation of aditi and rama of Vishnu, and devaki was incarnation of aditi and Krishna of Vishnu. Is the story of this promise found in Vedas?
It is hard to find such elaborate stuff in Vedas that generally deal with stories in a short sentence or passing reference. However, a similar conversation is present in the Srimad Bhagvat Purana
The section 1.10.3 Birth of Lord Krshna mentions:
Before actual arrival, Lord manifested Himself before Vasudev and Devaki. They felt as if a full moon had risen in the east. Vasudev saw that an extraordinarily marvellous baby boy was standing before them. His eyes were as soft as lotus; He had four arms and held a conch, mace, wheel and lotus. He bore Kaustubh Mani in the neck and wore yellow clothes. Valuable ornaments adorned all parts of his body. Thus, seeing God himself arrived as his son, Vasudev was overwhelmed. Devaki too was overjoyed but feared Kansa first. Then she joyfully prayed to God.
God said then, "During Swayambhu Manvantar, both of you had worshipped me with a desire of son. Vasudev was a Prajapati Sutapa while you were his wife Prishni. You had worshipped me with pure hearts. Pleased by your penance I had asked you to seek a boon, so you had sought a son like me in virtues. I had arrived then as your son Prishnigarbh. In the next birth you were Kashyap and Aditi and I was Upendra. Now, in the third birth also. I have arrived as your son."
This conversation tells us about the boon Aditi had received to get Lord Vishnu as her son though the Rama incarnation is not included in this.