Paul D. Kaplan, portfolio manager of the $24 billion Vanguard GNMA Fund and co-manager of the $38 billion Vanguard Wellington Fund, will retire on June 30, confirmed John Demming, spokesman for Vanguard Group. Thomas L. Pappas will become lead portfolio manager for the GNMA Fund; he has been on the fund's portfolio management team since 1994. John C. Keogh will take over the fixed-income portion of the Wellington Fund. Mr. Keogh is a senior vice president and partner at Wellington Management, subadviser to the fund, and has been a portfolio manager on the fund for two years. No decision has been made on a replacement for Mr. Keogh at Wellington.