It seems the forms caption is growing everytime it gets modified. You could try making the "base" caption a constant or assigned in an added form property(.mycaption)
In the forms init:
thisform.mycaption="My Base Caption"
thisform.caption=thisform.mycaption
In the caption procedure:
.q = alltrim(b)
.caption = .mycaption + " vendor "
.caption = .mycaption + .q


>while assign values to Form caption as follows:
>
>

>lparameters a, b && a = subkey passed, b=title
>with thisform
>	.q = alltrim(b)
>	.caption = .caption + " vendor "
>	.caption = .caption + .q
>	>

>
>I get following error:
>
>

>Error with FRM_SUBPAYSEL - Caption: String too long to fit
>

>
>Does Form caption have limits?
>
>thanks
>Nick