Does Trespasser il-vec still deal damage after giving itself Shadow?
Yes, Trespasser still deals combat damage.

The relevant passage is in comprehensive rules 509.1b (emphasis mine):


  A restriction may be created by an evasion ability (a static ability
  an attacking creature has that restricts what can block it). If an
  attacking creature gains or loses an evasion ability after a legal
  block has been declared, it doesn’t affect that block. Different
  evasion abilities are cumulative.