I suggest Open Babel. It is available on Linux, Windows and Mac operating systems and converts almost all molecular geometry file formats to each other.
Both solutions work very well for input files, but do not work for output files (please note that the *OUTCAR* file is mentioned in the question, not the POSCAR). I'm not aware of any tool enabling conversion of output files, at least not to the VASP format.
I admit I'm also a bit confused why you'd want to convert an output file, implying a completed calculation, of one program to that of another. Input, sure, but output?
I think Converting the output file of one(QE) to another(VASP) is not possible. If it is so, then no need to buy VASP by investing huge money. Why do you want to convert? Is there any specific reason?
May be the question is related to the conversion of optimized coordinates(from QE) to POSCAR (VASP). If it is so, follow the below instruction
1.Open your QE output file wiht XCRYSDEN
2. export optimized coordinates in xsf format
3. then open .xsf file with VESTA and export it in POSCAR format.