Are there any serious diseases you could get by talking to a vagabond in close proximity and shaking his hand? esp. diseases that kill in stealth?

No: Jimmy, you did a good thing, and you're a far better man than most of your peers. One of the most wonderful gifts you can give another human being is your time. A homeless person may suffer from loneliness and from mental bewilderment beyond what you can even imagine. You were surely a blessing to him. Don't worry -- you're tough, and your body can handle whatever was on his hands