In a large pot set over medium-high heat, heat the oil and butter. When the foam subsides add the onion, leek and garlic. Slowly sweat the vegetables until they're tender and soft but not browned - 5 minutes or so.
Add the parsnips and continue to cook, stirring often. After about 5 minutes, add the stock and bring to a simmer.
Cut the vanilla bean in half lengthwise and scrape out the black seeds. Add pod and seeds to the simmering soup. Simmer for 30 minutes until parsnips are tender.
Remove the vanilla pods and discard, and stir in the cream. Puree the soup well using a hand-held immersion blender or in batches in a regular blender. Add the lemon juice and season with salt.