A. successful / declared for: This option doesn't fit the context. The party is not looking "successful" due to members joining other parties, and "declared for" doesn't convey the idea of leaving the party.

B. vulnerable / defected to: This option fits the context well. The party looks "vulnerable" (in a weakened position) because many of its members have "defected to" other parties, indicating a loss of support.

C. cheerful / moved to: This option doesn't accurately convey the negative aspect of members leaving the party. "Cheerful" and "moved to" suggest a positive or neutral change, which is not consistent with the situation described.

D. cheerless / cross-carpeted from: "Cheerless" does convey a negative tone, but "cross-carpeted from" is not a common expression. "Defected to" is a more appropriate phrase to use in this context.

Therefore, option B is the most suitable choice as it effectively captures the idea that the party is vulnerable because its members have defected to other parties.