PKCS#7 message content I am using a vendor library that signs a text producing a PKCS#7 message. I'm not sure of the content of this message. 

Does a pkcs#7 message have to contain the signed message, the certificate, the payload? Does it have to start with ----BEGIN PKCS7---- and end with ----END PKCS7----?
First, PKCS#7 -- almost always considered to include its successor CMS in the same way "SSL" usually means TLS -- supports several cryptographic functions; you appear to be asking only about signing, so I will ignore the other parts. If you want more, read the RFC and/or add to your question.

PKCS#7/CMS has two options for signed data, one where the data is contained within the SignedData message, usually called embedded,
and one where the data is not in the message but must be available to the recipient, usually called separate, external, or detached.

PKCS#7/CMS messages including SignedData (but not necessarily external data) are first encoded in ASN.1 BER or DER 
which can then be transmitted and/or stored in binary or in PEM format using the dashed BEGIN,END PKCS7|CMS lines around a base64 encoding. (CMS messages 
can also be used in S/MIME messages with a slightly different formatting, but those are usually called S/MIME not PKCS#7 or CMS. In particular, a "clearsigned" S/MIME message contains a detached encoded signature plus a more-readable direct encoding of the data as parts of one email message.) 

A PKCS#7/CMS SignedData can carry one or more signatures, but more is rare; 
it can carry certificate(s) and/or CRL(s) that the signer believes the recipient(s) will or may need to verify the 
signature, but this is not required if the sender knows or believes the recipient(s) have or can get the certificate(s) and revocation info by other means.

In fact, PKCS#7/CMS SignedData can be used with no signatures and no data solely to carry some related certs (and possibly CRLs); 
this is commonly indicated as "p7b" or "p7c" format. This doesn't apply to your case, but could be confused with it.

You can determine which elements are present in a given message by parsing it; if particular, if you have or get 
OpenSSL openssl asn1parse will by default decode a PEM file (ignoring the type stated in the BEGIN,END lines, which can even be omitted) or with -inform der a binary DER file.