Are PSP allowed to send VOP requests when the intention to pay is deferred, for instance during vendor onboarding in a corporate ERP system?

The EPC cannot provide any legal advice about compliance with legal regulations. Each scheme participant and market operator (including RVMs) must conduct their own compliance evaluations.

What character set should be used in the VOP scheme API specifications and what about the special characters?

As mentioned in the VOP scheme API specifications, only the Latin characters set should be transported in the inter-PSP API messages, through UTF-8.  This restriction has been agreed in order to overcome the complexity that

Does a timestamp in the VOP API that contains trailing zeroes for milliseconds, lead to a rejection due to invalid format?

The VOP API specifications include a number of timestamp fields in the headers and body. The format of these timestamps does not allow for trailing zeroes for milliseconds.