Looking at rules for Magic and Essence in Run Faster (p.141). A vampire's maximum Magic attribute is their current Essence plus their Initiate Grade. If they let their Essence drop too low, they start losing points of Magic.
Say a vampire magician (not an initiate) with Magic 6 gets a little careless about his Essence, and lets it fall to Essence 5. By the rules in Run Faster, he also loses a point of Magic. Once he finds a victim to drain and replenishes his Essence, the point of Magic doesn't come back automatically. He has to "reconnect" the lost point by taking a geas, by draining extra Essence (beyond the limit of 12), or by spending 6 karma (only 1x the restored level, not 5x as if he were raising the attribute for the first time). Draining extra Essence seems the obvious best option, but it does trigger an addiction test.
Now, say a vampire grade-1 initiate spends 35 karma to raise his Magic Attribute to 7. Same situation. When his Essence drops down to 5, his Magic falls to 6 (max is current Essence plus Initiate Grade). After he refills his Essence tank, he can recover the lost point of Magic by any of the same three methods (geas, surplus Essence, or 7 karma).