the rights of each affected employee to benefits accrued to the date of such termination or partial termination (or, in the case of a plan to which section 412 does not apply, discontinuance), to the extent funded, or the rights of each employee to the amounts credited to his account at such time, are nonforfeitable (within the meaning of § 1.411(a)-4 .
For purposes of this subparagraph, the particular date of termination shall be the date of termination determined under section 4048 of such Act.
26 C.F.R. §1.411(d)-2
Sec. 411 (88 Stat. 901; 26 U.S.C. 411 )