BreakEven treshold - decision making

#1
If my break-even for response rate is 60% and total population 500,000

What would be the best way to determine if I should continue the vendor :

N sample size response

vendor 1 : 50 70%
vendor 2 : 2000 30 %
vendor 3 : 100 10 %
vendor 4 : 10 5%


What's the best approach to find if the sample size is big enough or significant enough to make a decision whether or not I should continue vendor ?

Is there any website I can use to calculate the value ?
How can I do it in excel ?

Thanks in advance