Robber A now knows the address of the vendor. Robber A can sit and wait until someone (Victim A) purchases something from this address and has a very large Bitcoin balance.

Most vendors would generate a new receiving address for each customer, as is recommended, which would stop this from happening. It is likely Victim A would also have generated multiple addresses rather than owning all his BTC on one address.

Robber B goes to random persons house with a gun and demands a Bitcoin transfer equal to Victim B's full wallet. Am I missing something because it seems incredible easy for people to do? Today cash, credit, and debit robberies have preventions/limitations in place to thwart very large robberies.

How can a cash robbery of exactly the same type be prevented? Although as above, it is unlikely the robber would know their entire amount of BTC owned, so they could not know if the victim had sent everything or not. In general you could keep large amounts of BTC stored on encrypted cold wallets off-site which any robber should not know anything about.