Place quince, vanilla pod, and lemon peel in large saucepan; cover with water. Bring to boil, then simmer covered until quince fork-tender (30–40 minutes).
Drain; discard vanilla pod (keep peel). Purée fruit with peel. Measure purée volume; add equal volume sugar and lemon juice in pan over medium-low heat.
Cook, stirring occasionally, until very thick and deep orange-pink (1–1½ hours).
Preheat oven to 52°C / 125°F. Line 20 cm (8 in) square pan with greased parchment. Spread paste evenly; dry in oven ~1 hour. Cool completely; slice.