Conversion operator causing compilation error

I have implemented a string-like class, called Str, and want to add a conversion to bool type, in order to use a Str object as a condition. The Str class is quite simple and relies on the STL vector class. Everything works fine, except that now, if I try for example to do the following:

Str greeting = "Hello, " + name + "!";

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where "name" is a Str, then I get a compilation error, saying "'operator +': 2 overloads have similar conversions, could be 'Str operator +(const Str &,const Str &)' or 'built-in C++ operator+(const char [8], int)'.

I understand the reason for the error, but I am not sure how to fix it. How can I get around this? Below I have copied a minimal working example.

#include <iterator>
#include <vector>
#include<iostream>

using namespace std;

class Str
{

private:
    std::vector<char> data;

public:
    typedef std::vector<char>::size_type size_type;
    typedef std::vector<char>::iterator iterator;
    typedef std::vector<char>::const_iterator const_iterator;

    iterator begin() { return data.begin(); }
    const_iterator begin() const { return data.begin(); }
    iterator end() { return data.end(); }
    const_iterator end() const { return data.end(); }


    // default constructor must be defined explicitly, since non-default constructors are also defined
    Str() { }
    Str(const size_type n, const char c) : data(n, c) {  }
    Str(const char* cp)
    {
        std::copy(cp, cp + std::strlen(cp), std::back_inserter(data));
    }
    // Since this is a template, the iterators can be anything: pointers in an array of chars, std::vector<string> iterators, std::std::vectortor<string> iterators, std::list<string> iterators, etc.
    template <class In> Str(In b, In e)
    {
        std::copy(b, e, std::back_inserter(data));
    }
    char& operator[](size_type i) { return data[i]; }
    const char& operator[](size_type i) const { return data[i]; }
    Str& operator+=(const Str& s)
    {
        std::copy(s.data.begin(), s.data.end(), std::back_inserter(data));
        return *this;
    }
    size_type size() const
    {
        return data.size();
    }

    // conversion operators
    operator std::vector<char>() const;
    operator bool() const;

    template <class In> void insert(iterator dest, In b, In e)
    {
        data.insert(dest, b, e);
    }

};

Str operator+(const Str& s1, const Str& s2)
{
    Str s = s1;
    s += s2;
    return s;
}

Str::operator std::vector<char>() const
{
    std::vector<char> ret = data;
    return ret;

}

Str::operator bool() const
{
    if (data.size() > 0)
        return true;
    return false;
}


int main()

{

    Str name = "Joe";
    Str greeting = "Hello, " + name + "!";

    return 0;
}

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