I've never heard or seen the word "vission".
A mission is an important duty or work that has to be done. Often used in context with the military. The word "job" could be a synonym for it.
"Your mission is to eliminate the target."
"I have to finish this assignment today. It is my mission!" (suggesting the seriousness and urgency of the situation)
Vision means sight. The ability to see.
It can also mean things that you see in your head or imagination. The future, an idea coming to life, or an experience/dream that you experienced very vividly or felt very real.
"Her vision was blurred by tears."
"I had a vision about today's meeting."