If your vendor is using a website to collect money you can simply create a 'payment button' for your account wich they can place on their website. No need for API. All payments will be made to your PayPal account and they can't access anything.
We want them to be able to collect money and put it in our paypal account
If you do it with this 'payment button' the highlighted will be automated since it is direcly stored toward your paypal account.
- For a 'Payment Button' read here
https://www.paypal.com/us/cgi-bin/webscr?cmd=_singleitem-intro-outside
for a 'payment button'. Remember that payments can be refunded.
If you want people to 'donate' since the vendor is collecting money you can better use a 'donate button' found here https://www.paypal.com/us/cgi-bin/webscr?cmd=_donate-intro-outside
since people cannot refund their donation once they transferred the money.