Creating financial accounts adds risk, and maintenance. People naturally a hesitant to creating more.
If one person has only a PayPal account, and wants to pay someone else who has only a Venmo account, can it be done?
Short answer: No.
From the Venmo Payments & Requests FAQ:
Although Venmo is a service of PayPal, Inc. we do not offer the ability to transfer funds between Venmo and PayPal accounts at this time.
Seeing as how the two payment methods are linked, the ability to send a payment to a Venmo account from a PayPal account is hopefully not too far off.