I have two questions ....

1. I could not help seeing some of my younger brother's lifelong admiration for me getting siphoned off to Philbert.

I understand the use and the meaning of siphoned off here. My question is "Is this verbal phrase commonly used in spoken modern English?? may you give me examples (I don't know why I feel it's a little bit vulgar???Is it right??)

2. I feel down to earth. May you tell me the meaning?

Siphon off is not vulgar. It may not be common in spoken English, but it is not vulgar. There are 379 thousand hits in Google with plenty of examples for the use of "siphon off" in writing and in speech.

down-to-earth adj.

1. Realistic; sensible.

2. a. Not pretentious or affected; straightforward.

b. Not overly ornate; simple in style.

The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company.
It is not a verbal phrase, siphon off. It is a phrasal verb consisting of a intransitive verb and an adverbial particle.
Siphon off is informal.
Down to earth means you have a realistic appreciation of things around you. If you are down to earth you are sensible.

One more observation: "May you tell me" sounds very odd to me. I think 'may' is ill-used this way.
Never in my life have I met 'may' used this way.
 Likeguslee's reply was promoted to an answer.