Meanwhile on Martha’s Vineyard, everything is ready for the second coming of the summer White House. The dirt road to the seaside compound that President and Mrs. Clinton plan to visit is freshly graded. Telephone crews have laid miles of new cable, and a couple have been booted from an adjacent farmhouse to make room for presidential staff. But since Clinton vowed not to leave Washington with Congress in session, the First Family may be lucky to get there for Labor Day weekend.