Two quick points to consider:
1. You cannot mix very and more. Very provides an empasis, strength, intensity to whatever it describes. More indicates that some is greater, additional, or further (per the Merriam-Webster dictionary).
2. The concept of "more" can be incorporated into a word by appending "-er" to the end of the word. So the concept of being "more happy" is: happier. The concept of being "more sad" is: sadder. The concept of being "more fast" is: faster. In all of these cases, the "more" version is incorrect because there is an "-er" version that means the same thing.
Anytime that a word can be made "more" by adding "-er" to the end, you cannot combine it with the word more. In other words, you cannot say more happy, more sad, or more fast. Rather, you must say happier, sadder, or faster.
If a word cannot be made "more" by adding "-er" to the end, then you can often combine it with the word more. For example, you can say more amazed, more interesting, or more horizontal. You cannot say amazeder, interestinger, or horizontaler.
There are always exceptions, but I hope that this helps.