The fixed order in the algebraic method (AM) suggested by Sun et al. is changed to be a flexible one in the vibrational energy expansion because the order of diatomic potential energy expansion may not be a constant. The AM with a flexible order was used to tackle the possible “butterfly effect” that may be encountered in spectroscopic computations, and to study the full vibrational levels ｛Ｅν｝ and the dissociation energies Dｅ for Ｎ２－ａ′１Σ－ｕ, Ｌｉ＋2－２ ２Σ＋ｇ, ４ＨｅＤ＋－Ｘ 1Σ＋ and ３９Ｋ ８５Ｒｂ－(２)３Σ＋ electronic systems. The results reproduced all known experimental vibrational energies, and predicted correct dissociation energies and all unknown high-lying levels that may not be given if one uses original AM. The calculations showed that the modified AM can be extended to study the full vibrational spectra for many more diatomic systems.