Not sure if you had any answer choices, but through merchants, who traded across the Indian Ocean, Hinduism spread to Southeast Asia, in regions like Vietnam. Buddhism spread along trade routes like the Silk Road. It made its way into East and Southeast Asia in regions like China, Korea and Japan. If you are looking for an area they both spread to it would be Southeast Asia.
As a muslim, I can positively say that Islam does have the pre-Islamic Arabian influences, such as loose clothing to prevent the body's curves from showing, not consuming alcohol or eating pork of obtaining animal's blood. However, there are practices that are different. Such as believing in one God, not worshipping any idol, object, or person. Believing God have no children or a wife, neither does he have any family. He is one and only, no one is like him. Not talking to a non-marram, or someone who's not your close family such as cousins, uncles, aunts, husband , or wife. Not taking intrest, since that's not your own earned money. Also praying five times a day and fasting in Ramadan. P,u'd giving as much charity as possible. If you have any other questions, feel free to ask.
