YOUR FUTURE: WHAT TO DO TO GO BEYOND VECTOR CALCULUS
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The obvious next 3 topics to learn (in any order - you
should now be ready for any of these) are

* Linear algebra 
is about matrices instead of just vectors. A huge area.
recommend books: 
Golub & Van Loan: Matrix computations
Horn and Johnson: Matrix analysis

* Calculus of variations
instead of functions of, say, 3 variables, how about functions of an
INFINITE number of variables (such as: a "function of a function")
and how to optimize THEM (such as, by choosing a function of x, which
is the best possible curve in the world for some purpose).

* Differential equations - this is pretty much single variable
  calculus again, but rates of change of various things are related by
  "differential equations" and you want to solve them to find out
  what is going on.
recommend book:
Ince: Differential equations, Dover.