Upon a determination, by resolution, of the governing body of the community in which such land is located that the acquisition and development of undeveloped vacant land, not within a slum or blighted area, is essential to the proper clearance or redevelopment of slum or blighted areas or a necessary part of the general slum clearance program of the community, the acquisition, planning, preparation for development or disposal of such land shall constitute a redevelopment project which may be undertaken by the authority in the manner provided in the foregoing sections. The determination by the governing body shall be in lieu of the declaration required by section six-b above but shall not be made until the governing body finds that there is a shortage of decent, safe and sanitary housing in the community; that such undeveloped vacant land will be developed for predominantly residential uses; and that the provision of dwelling accommodations on such undeveloped vacant land is necessary to accomplish the relocation, in decent, safe and sanitary housing in the community, of families to be displaced from slum or blighted areas which are to be redeveloped: Provided, however, That in the undertaking of redevelopment projects on a regional or unified metropolitan basis, involving the acquisition and development of undeveloped vacant land in one community as an adjunct to the redevelopment of slum or blighted areas in another community, each determination or finding required in this subsection shall be made by the governing body of the community with respect to which the determination or finding relates.