Not exactly sure what you're trying to achieve, but you can get the pointer to the memory used by the vector (or matrix) via the .memptr() function. This pointer can then be used, along with an offset, to create a new vector (or matrix) which uses external/auxiliary memory through dedicated vector constructors and matrix constructors.
For example:
vec v1(35000000);
vec v2(v1.memptr(), 17500000, false); // v2 will now share memory with v1
Incidentally, don't use the .resize() function with Armadillo vectors and matrices unless you really mean to preserve existing data. Use .set_size() instead, which is much faster.