My dear, first of all, u must know that lord krishna existed on the earth which has been proved by the archeologists. Dwarkapuri has been discovered deep inside the see whose design matches with those written in mahabharata. So many authentic materials have been discovered which give a solid proof of the lord's existence. Second thing is that if you go through the mahabharata, you will find that whole war was carried away by lord krishna, but sage vyasa documented the war in a single epic, so, there is nothing wrong in it to say that geeta is krishna's words which was documented in a single book by ved vyasa. And if the matter is of lord vishnu in vedas, you are correct that vedas have said a little about vishnu but in these little hymns only, he has been given the supremity and there is nothing wrong in it, you can go through rig veda and find that its true. Archeologists have also revealed that there has been a war around 3000 bc in kurushetras, bones have been discovered from the land which are emitting nuclear radiations still which proves that devastating weapons were used there. All these have been discovered through carbon dating. So, your assumption is wrong that krishna is not historical.
