1-how many addresses are there with this characteristic?
Depends on what you mean
- The total funded ones?
- The total they can exist?
1- what are the probabilities of finding this type of address?
Well, that's some tough calculation that I can't do at the moment, I may check it tomorrow. You may get close to the answer if you generate hundreds of thousands of addresses and divide the ones who do not contain numbers in their base58Check with the total ones you generated.
Now, answering to both:
3- as these types of addresses are considerably strange and proportionally in less quantity, does my code make them vulnerable?
Yes in theory if you are running any of the brute forcing key cracking software it will get there sooner.
This is so not true! There's nothing wrong with these addresses. Their base58 encoding just didn't happen to return numbers. They're as secure they all are.
Whether that's 1LdRcdxfbSnmCYYNdeYpUnztiYzVfBEQeC, 1111111111111111111114oLvT2 (lowest) or 1QLbz7JHiBTspS962RLKV8GndWFwi5j6Qr (highest). Remember, they are just RIPEMD-160 hashes represented in a special format.