How can i create a istream from a uint8_t vector?

std::basic_istream gets its data from an associated std::basic_streambuf derived class. The STL provides such classes for file I/O and string I/O, but not for memory I/O or network I/O.

You could easily write (or find a 3rd party) memory-based streambuf class that uses the std::vector as its underlying buffer, and then you can construct an std::istream that uses that memory streambuf. For example (using the imemstream class from this answer):

std::vector<uint8_t> &data = networkMessage.data;
imemstream stream(reinterpret_cast<const char*>(data.data()), data.size());
processData(stream);

Well C++ does actually have a class for this - istrstream, and you could use it like this:

    vector<uint8_t> data = ...;

    // need some way to create istream from data
    std::istrstream stream(reinterpret_cast<const char*>(data.data()), data.size());

    processData(stream);

As far as I can tell this doesn't copy the data, unlike the other answers. However it was also deprecated in C++98 because it's hard to know who is responsible for freeing the buffer, so you may want to write your own equivalent.