Transistor as switch in variable voltage environment
Your question sounds ALMOST clear but it's not 100% certain what the perceived problem is.

You seem to be saying that the sensor will run from the minimum Arduino supply directly, but that adding a transistor in series with it may reduce the sensor to below allowable. 

An on bipolar transistor can have a saturation voltage when on of about 0.1V without too much effort.  More base drive (ie more base current) will usually give a lower on voltage. A beta (current gain if 10 - ie Ic/Ib = 10 will usually give you sensibly as low a Vce as you are liable to get in most applications. You do not say whata sensor current is but it is unlikely to be high. Possibly &lt; 1 mA, unlikely to be > 10 mA. Any high current gain jellybean NPN should do reasonably well. I favour the BC337-40 (or SMD equivalent BC817-40) in this sort of application but many transistors will be OK.

Lowest Von is liable to be achieved with a MOSFET. You can get Rdson lower or muh lower or much much lower than 0.1 Ohm if desired. At say 10 mA load and Rdson = 100 milliOhm  then Vds = I x R = 10 mA x 0.1 Ohm = 1 milli Volt !!!!!!!!!!!!!!!!!!!!!
Thatshould be low enough :-).

If you decided to use a bipolar transistor and decided to use a Beta (current gain) of 10 (will be much larger in practise) gives (for Iload = 10 mA. Ibase = Ib/min Beta = 10/10 = 1 mA.
Rb = V/i =  (Vdrive - Vbe) / 1 mA = (3.3-0.7) / 0.001 &lt;= 26k.
Rb = 22K will work well.

Ioff will be close to zero.