Here's a simple way to think of it...
Voluntary resale = just get to use what you bought or pay for an account to exchange it in II, no access to VSN (StarOptions for internal trades to Vistana properties), no VSN fee to pay, no Abound access.
Mandatory resale = receive VSN access/ StarOptions/pay VSN membership dues, use what you bought or exchange it through II in your free Vistana II account, use StarOptions trade to other Vistana properties 8 months out, pay all transaction fees in Vistana and a reduced II exchange fee of $164 if trading to another Vistana or Marriott property, no Abound access.
Direct purchase from Vistana = everything you get with mandatory resale plus Abound access to access Marriott timeshare properties with your Vistana ownership, pay Abound club dues instead of VSN dues, get all of your Vistana transaction fees waived plus free trades/retrades in II to other Vistana and Marriott properties.
You can also buy under one of the resale options and then do an additional direct purchase from Vistana of a certain dollar amount to make the resale have all the same benefits of a direct purchase.