I run into old fixtures like this with no box all the time.
If its romex or knob and tube you typically dont have enough wire to add an exterior bell box.
I usually carefully cut in a 3/0 or 4/0 old work box like one of these:
4 in. Round Nonmetallic Vapor Barrier Ceiling Box, 2-39/64 in. Deep
www.hubbell.com
But with vinyl siding I'd be tempted to use a
For smooth vertical mounting of light fixtures on all siding types. Kit includes mounting block with built in electrical box, NM cable connector, and 4 screws. Paintable, UV rated plastic for long outdoor life. All boxes except 8141DBL and 8141DBLGC have a 6.6" x 6.6" octagonal box. 8141DBL has...
www.aifittings.com
WAC 296-46B-901
(7) As required by chapter
19.28 RCW or this chapter, an electrical work permit is required for the installation, alteration, or maintenance of all electrical systems or equipment except for:
(b) Class A basic electrical work which includes:
(i) The like-in-kind replacement of lamps; a single set of fuses; a single battery smaller than 150 amp hour; contactors, relays, timers, starters, circuit boards, or similar control components; one household appliance; circuit breakers;
single-family residential luminaires and line voltage smoke or carbon monoxide alarms; a maximum of five snap switches, dimmers, receptacle outlets, thermostats, heating elements, luminaire ballasts or drivers/power supplies for single LED luminaires with an exact same ballast or driver/power supply; component(s) of electric signs, outline lighting, or skeleton neon tubing when replaced on-site by an appropriate electrical contractor and when the sign, outline lighting or skeleton neon tubing electrical system is not modified; one ten horsepower or smaller motor.