I am planning to take Multivariable calculus in senior year; will this course stand out to Ivy League admission officers?
Very common for students who take Calc BC in 11th to take MVC in 12th. So, no.
About a quarter of Harvard’s students have had MVC or beyond in HS, so, no
https://features.thecrimson.com/2018/freshman-survey/academics/
My kid’s senior year, about 35-40 kids took MV Calc, with a class size of about 525 students. Relatively common.
It demonstrates course rigor, certainly. Though it is only one factor of many in your application.