As a vendor, can I just send my bitcoins I receive from my sales to another account on a different market (with the intentions of making a purchase). Or is tumbling always necessary despite the fact I am not trying to convert my btc to cash or hold onto it in my actual wallet?
Also do vendors usually use multiple tumblers (helix, bit blender, etc.) when they have large amounts of btc they want to transfer to their personal wallets?
As long as you are not planning to sell your Bitcoins at an exchange there is no need to tumble them.
If you are sending your Bitcoins to your Electrum wallet for example, you won't need to do anything because your Electrum wallet isn't associated with your identity.
If you are going to cash out your Bitcoins at an exchange that has KYC, you will need to do something to make sure your Bitcoins aren't easily linked to your darknet adventures.
Every vendor probably has a different way to deal with this. Some might use multiple tumblers like you suggested, some vendors don't care and don't do anything at all, some tumble their coins once and sell them trough LocalBitcoins for cash, some might trade them for altcoins like Monero and will then sell those. And there are many more ways to sell your Bitcoins anonymous. There is not one way to do this, you will have to be creative. :)