Growing your own vanilla isn't difficult, but it requires more greenhouse space than you might imagine. The plant grows like a vine, and it'll need to reach about 20' with multiple points of ground contact before producing pods. (as explained to me by an orchid enthusiast)
Growing your own vanilla is very difficult (assuming you're not in a place where it grows outside naturally). Not only do you need a significant amount of greenhouse space (you're right--20' minimum to flower, but the vines can grow up to 120'), you also need all the very specific orchid growing conditions--low water/nutrient, high drainage soil (or bark or similar medium), high brightness (but medium direct sun--can't let 'em burn), along with a very high humidity (vanilla orchids grow best ~80-90% humidity) and high heat (ideally not dipping any lower than 15 C, but with a bit of wiggle room). They have aerial roots, so you also need to mist with diluted nutrient mix (preferably an x-x-x fertilizer (eg: 20-20-20)) for them to get any particular growth benefit.
Assuming you can provide these conditions, you'll have to continue to provide these conditions for ~5-7 years, which is how long vanilla takes to grow from seed. Most people, however, take clippings from already healthy plants. Clippings need all the same care and effort of growing from seed, but won't flower 'til they're established and ~20 feet long (and the temperature controls are on-point for flowering), so how long they take depends on a bunch of factors, but even for a 3' clipping, you're looking at ~5 years.
So, let's say you have flowering-capable vines, at least 20', properly cared for with appropriate temperature controls to compel flowering. The vanilla flowers for one day. Furthermore, the pollination of the one-day flower must occur by hand (you cannot use natural pollinators--there's only one species of bee capable of pollinating this thing and they're not only isolated to a very specific geographic location, but they're also rather rare).
Presuming your pollination is successful it still takes, at this about, about 9-10 months for the vanilla pods to form. You'll get long green-bean lookin' things. You have to pick them as they begin to turn yellow--kinda like bananas. But the fun doesn't stop here. At this point you still have to blanche the yellowing beans, then let them sweat in a container for several weeks until you finally end up with usable vanilla pods that you might find in a store.
The process is extremely involved, incredibly time consuming and each step has a number of 'gotchas' that will ruin your vine/flowering/pollination/beans. The big effort is literally just keeping up with the very specific humidity/light/temperature/nutrient requirements for so long to even get it to be flowering capable.