Depending on your definition of SNR, you have different options in LabVIEW.
If you consider distortion to be part of "noise" then the SINAD (Sine in Noise and Distortion) VI is doing exactly what you need. The VI computes the ratio between the energy of your signal and the energy of the residual signal (after your tone has been removed).
If you are interested in the ratio between signal and true noise (no harmonics), then you can, for example use the Harmonic Distortion VI to extract respectively the Fundamental signal, the Residual Signal and the Harmonic only signal and from there compute your "Signal to (residual-harmonic)" ratio.
Another more empirical method is to perform a Power Spectrum measurement on your time signal and then evaluate the level of your (white) noise floor. Then compensate for the bandwidth of interest and for the noise bandwidth of the used window and compute your SNR from there.
All the required VIs are available from your Analyze >> Waveform Measurement palette (LabVIEW 6i or higher required).
