This link will tell you everything that you need to know ...
http://www.lmgtfy.com/?q=nfhs+%22pole+vault%22+rulesThis will get you to the National PV Coaches Association website (the top Google entry on the list).
From there, read ART. 7-9 in Section 5 Pole Vault.
To answer your specific question re the top mat, ART. 7 says in part ...
When the landing system is made up of two or more sections, the landing surface shall include a common cover or pad extending over all sections.
... and ART. 8 says in part ...
The front pad shall be attached to the main landing pad or encased in a common cover.
Besides your question about the pit, pay particular attention to ART. 9 ...
ART. 9… Hard or unyielding surfaces, such as but not limited to concrete, metal, wood, or asphalt around the landing pad, or between the planting box and the landing system, shall be padded or cushioned with a minimum of 2 inches (50mm) of dense foam or other suitable material(s).
Note: It is recommended that any excess material such as asphalt or concrete that extends out from beneath the landing pad be removed.
I've seen several pits in both Canada and the US where this rule is broken. It's one of my pet peeves, since adherence to this rule (if it existed back in my vaulting days) would have saved me my two front teeth and a concussion.
It's a FIRM RULE, so it must be complied with!
Kirk