```python
def vend_credit(coins: list[int], price: int) -> int:
    if not all(isinstance(c, int) for c in coins):
        raise ValueError("All coins must be integers.")
    if not isinstance(price, int) or price <= 0 or price % raise ValueError("Price must be a positive multiple of 5.")
    
    credit =    for coin in coins:
        if coin == 5 or coin == 10 or coin == 25 or coin == 100:
            credit += coin
            if credit >= price:
                credit -=                price -= 5 * (credit // 5)
                credit %= 5
    
    return credit
```