Sri Aurobindo
Letters of Sri Aurobindo
Volume IV - Part 2
It is not necessary to be so careful as all that1. Ordinary vital interchanges are of a slight character. Nobody can take away another’s vital, for the very good reason that if that happened, the person from whom it was taken would die. It is possible of course for one person to drain another’s vital forces so as to leave him limp or weak or dry, but it is only the vampire kind 
that do that. It is possible also for one to give out too much of one’s vital forces so as to weaken oneself or exhaust of energy, a thing which should not be done,– it is only those who know how to draw or can draw freely from the universal vital Force and replenish their life energies that can give out freely. All of course draw to some extent, otherwise they would not remain alive, for expenditure of vital energy is always going on and one has to replace it; but for most the capacity for drawing is limited and the capacity for giving without exhaustion is also limited.
But the ordinary movements of interchange are harmless provided they are kept within moderate limits. What creates a difficulty in the sadhana is that one may easily draw in undesirable influences or pass them on to others. It is the reason why at certain stages a limitation of talk, intercourse etc. is often advisable. But the true remedy is to become inwardly conscious, to know and be able to repel any undesirable incursion or influence, to be able when speaking, mixing etc. to keep a defence round one and allow to pass in only what one can accept and nothing else. Also to measure what one can give out safely and what one cannot. When one has the consciousness and the practice, this working becomes almost automatic.