How much shear load can rebar withstand?
One of the 1/2” rebar “you have access to” has enough “shear strength” to support my car full of groceries with me in it. 

However, you don’t want to use it for a horizontal support of grape vines, because it will rust and deposit so much rust in your vineyard that you’ll be labeled a hazardous waste site. 

Also, that rust will contaminate the taste and bouquet of that wine so much that I’d have to hold my nose to drink the wine. 

Try something a little more common like wire.