Improving on insert()

Your spider senses are tingling for good reasons. std::vector's insert() function gets two iterators that describe the range to copy from, but if it's written in a generic way it cannot make any assumptions about those iterators, so it might just copy character by character instead of doing a call to memcpy(). Also, every time it adds another character, it has to make sure Buf is large enough, and reallocate its storage if not.

However, it can in principle detect what you are trying to do and implement an optimized version that basically just reallocates and then memcpy()s. Some compilers do this, while others don't. You could "help" the compiler by making this explicit; instead of calling insert(), write:

auto old_size = Buf.size();
Buf.resize(old_size + Random.getName().size());
std::copy_n(Ramdom.getName().cbegin(), Ramdom.getName().size(),
            Buf.begin() + old_size);

However, that might also generate worse output, as resize() might cause value-initialization of the extra storage being added, even though it will be overwritten by the call to std::copy_n(). It all depends on how the compiler optimizes your code.

While the above shows how you could change a single call to insert(), consider that you could also calculate the size of everything you are going to add up-front, then do a single resize() in Serialize(), and then just copy all the data to the right place inside Buf.

If you want to write reasonably performant, maintainable code, then I would keep the calls to insert() as you have now. If you need to get every bit of performance you can and maintainability is of lesser concern, then try the above, create realistic performance benchmarks, and measure this on several platforms with different compilers to see if it's worth it or not.

Call reserve() if possible

Regardless of how you add elements to vectors, if you have some idea of how much you are going to add, you can call reserve() first. This might avoid multiple memory (re)allocations later on.

Avoid code duplication

Your code basically does the same thing three times: add a size to a buffer and then copy the data. Consider writing a function that does that:

template<typename T>
static void append_data(std::vector<uint8_t> &Buf, const T &Data)
{
    auto EncodedSize = EncodeU32(Data.size());
    Buf.insert(Buf.end(), EncodedSize.begin(), EncodedSize.end());
    Buf.insert(Buf.end(), Data.cbegin(), Data.cend());
}

std::vector<uint8_t> Serialize(Ramdomclass &Ramdom)
{
    std::vector<uint8_t> Buf;
    append_data(Buf, Ramdom.getName());
    append_data(Buf, Ramdom.getContent());

    std::vector<uint8_t> Result;
    Result.push_back(0x0);
    append_data(Result, Buf);

    return Result;
}

Alternative ways to serialize data

There are other approaches to serialize data that you could use, or perhaps look at to get inspiration for your own serialization code.

If you want to have a clean and extensible way to serialize your own data structures in C++, then Boost::Serialization might be interesting for you. The drawback is that it doesn't do any compression of integers as far as I am aware.

Instead of a custom serialization format, you could use a standard format. For example, MessagePack seems like a good fit here, but there are many more.