Hello Robbie,
I think you are confusing two different things (very easy to do-I am not saying that I was confused also 🙂
But vlan 20 is just the vlan identification value. vlan.20 is an interface all be it an internal logical Layer 3 interface, just like ge-0/0/1 or fe-0/0/2, at-x.x.x, gr-x.x.x etc. Soif you have multiple interfaces in the vlan 20, you would need to add each of the interfaces in the vlan for them to begin sending and responding to OSPF hello packets. Adding vlan.x does not add all the interfaces in vlan X, that is a single L3 routign interface.
https://www.juniper.net/techpubs/en_US/junos12.1/topics/example/ospf-single-area-configuring.html
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To enable OSPF, you must configure one or more interfaces on the device within an OSPF area. Once the interfaces are configured, OSPF LSAs are transmitted on all OSPF-enabled interfaces, and the network topology is shared throughout the network.
did you run the command
>show ospf interface
How many interfaces did it show you that is running ospf?
here are some documents that that will help you understand OSPF
https://www.juniper.net/techpubs/en_US/junos12.1/information-products/pathway-pages/config-guide-routing/config-guide-ospf.html#overview