The algebraic method (AM) suggested by Sun et. al. is developed into a nodal variational AM (VAM) to offset the possible experimental errors by using an energy variational part δE after analyzing the error amplification effect. The VAM is used to study the full vibrational levels {Eυ} and the dissociation energies De for 51Πu7Li2, (6d)1Δg Na2, (7d)1ΔgNa2 and 51∑+ NaK alkali metal diatomic molecular electronic systems. The results reproduce all known experimental vibrational energies, predict correct dissociation energies and all unknown high-lying levels that may not be given if one uses original AM or other numerical methods or experimental methods. These theoretical analyses and results not only show that the VAM is feasible and correct for many diatomic systems, but also provide constructive reference for other numerical calculations or simulations.