1. A method for motion vector predictor derivation of a block, comprising: during a same-reference-frame stage of the motion vector predictor derivation, scanning a plurality of candidate motion vector predictors derived from neighbors of the block, and generating a determination result by determining if any candidate motion vector predictor scanned in the same-reference-frame stage is qualified for a different-reference-frame stage of the motion vector predictor derivation; and referring to the determination result to selectively enable the different-reference-frame stage following the same-reference-frame stage; wherein when a predictor list is not filled with valid reference motion vector predictors found by the same-reference-frame stage after the same-reference-frame stage is complete, and the determination result indicates that at least one candidate motion vector predictor scanned in the same-reference-frame stage is not qualified for the different-reference-frame stage following the same-reference-frame stage, scanning said at least one candidate motion vector predictor in the different-reference-frame stage is skipped.