Can anyone tell me what this means? The only definition I could find was "summer vacation" which made no sense in the context. This is what's on ether side of it:
nounになってる = nounになっている
has become noun
You need to be careful to distinguish between the large and small characters. It's なってる not なつてる and ポジション = "position", not ポジシヨン.
I have no idea what a "mount position" is, but it (これ）has become one. Hopefully you can figure that out from the context of the story.