Why do cells in meristematic tissues lack vacuoles?
Meristematic tissues or meristems contain cells which can rapidly divide or reproduce and is responsible for the overall growth of the plant. Apical meristems and lateral meristems are found in the roots and stems of the plants. Please note that the whole stem or root is not laded with these tissues. Plant cells usually have a large central vacuole which applies turgor pressure to keep the plant upright, as mentioned in one of your sources.
The tips do not require to be very stiff and straight. So, a central vacuole is just not required. Also, meristematic cells grow various organelles as they mature.