Drugs and alcohol and other non-necessities generally are not free. So I am not sure how you’d fund those things or the partying, depending what that means to you.
But if you choose to do that nothing would happen, society would support the costs of you having safe (although likely fairly frugal) housing, sufficient food, and medical care. There are already people who live that way but without the food, housing, or care; they may resort to crime to meet their needs (negative affect on society) or may just have such poor health that they require much more expensive care (already at the cost of taxpayer in capitalistic society) than they would have if they’d been able to access care before it became a dire emergency.
Research generally indicates that capitalist societies already pay more to address the consequences of poverty than it would cost to provide those people with their basic needs in the first place.
The answer is bison
But to the problem of converting to veganism on a larger scale, I think you’d just gradually phase out by banning further breeding and the cow population could dwindle down. It wouldn’t solve a lot of the other adjacent problems though.