Try this
http://www.s-ohe.com/Butane_cal.html

I also had info from another source giving the vapor pressure equation that seemed to compare well

n-BUTANE
Temperature range,  273.00K -  343.00K, Vapor Pressure, Pa

Y = exp(A + B/T + C*ln(T) + D*T^E) (SI Units)
A=  67.164
B= -4386.2
C= -7.1805
D=  9.7145E-06
E=  2.0000

With the above equation, I get
6C, 18.65 psi
50C, 71.74 psi

Libero:

The answers are:

At 6 oC, P = 18.727 psia
At 50 oC, P = 71.976 psia

This information is as reverent and accurate as the US government gets it and gives it FREE to you and all other interested parties that go to:

http://webbook.nist.gov/chemistry/fluid/

There, you will find full, complete, and accurate Thermodynamic properties on 34 fluids, including Butane.  You can select any unit you like and obtain HTML tables that can be copied and downloaded directly into an Excel spreadsheet.  What I like to do is take this tabular data and regress it – much like I have done with your two points:

Y = a*X^3+b*X^2+c*X+d

where,
a = 6.7289 E-006
b = 0.007971
c = 0.572973
d = 14.98395

Now, you got an accurate vapor pressure curve for Butane that you can use in computer program or a spreadsheet for everyday computations.  The accuracy of my equation will drop your jaw.  Free NIST thermo data is the best way to fly.

Thank you guys

Cheers