What is the maximum interception angle for ILS when on radar vectoring?
45 degrees.

8.9.3.6
Aircraft vectored for final approach should be given a heading or a series of headings calculated to close with the final approach track. The final vector shall enable the aircraft to be established in level flight on the final approach track prior to intercepting the specified or nominal glide path if an MLS, ILS or radar approach is to be made, and should provide an intercept angle with the final approach track of 45 degrees or less.

ICAO Doc 4444 (PANS-ATM)
That said, we very often use a 30 degree intercept angle, even if we are allowed to go up to 45 degrees.