The key to making this work is the configuration of the WAN list and the NAT rule. The short version is this:
/interface list member
add interface=your_PPPoE_connection list=WAN
add interface=your_vlan list=WAN
where "your_PPPoE_connection" and "your_vlan" are the identities of your PPPoE connection and VLAN as named in the list of interfaces. This makes sure that both these are in the WAN list that is then used in the NAT rule:
/ip firewall nat
add action=masquerade chain=srcnat out-interface-list=WAN
Which passes the info between the PPPoE connection and your local network. Before I added the VLAN to the WAN list, I could get onto google.com and a few other website, but only at low speeds. It was all very strange!
All that might only make sense once you know what to do anyway. Here are some screenshots using webfig
1) The list of interfaces. The PPPoE _Client and VLAN (listed under Type) are things you have to configure according to the needs of your ISP. For me on and NBN connection to Leaptel, the VLAN ID was 100.

2) Edit your WAN list by Adding New and adding both the VLAN and PPPoE interfaces.

3) Now add that WAN list to the NAT rule. The NAT rules are found in the Firewall options. I've highlighted the rule that we need to make.

4) The rule looks like this.
