Hi, since I have experience in C and C++, this project shouldn't be a big problem for me.
things like classes, vectors and templates can be done in C too. Depending on how you want the final C code to look like, it is possible to implement something like c++ vector in C, if it should have just the same functionality like the original c++ code it is aswell possible to use plain arrays, which would be a performance advantage. classes can be implemented as C structs with function pointers inside, or again, if it should just have the same functionality, as functions operating on data sets(structs).
Templates in C++ are a preprocessor task, which in C could be done with ether macros, if you want to have the same generic look to it, or just by implementing the needed functions for the types or in the shape that you need(that would mean to write methods or functions created by C++ in compile-time by hand in C).
Since i have some free time right now, and i need some first projects on this site, I would like to do this project for you. The time i need for that depends on the size of the C++ program that i should convert, so the project time could be longer or shorter than 5 days depending on your program.
If you are interested, just send me the details, and i can start working.
best regards.